Wednesday, 31 December 2014

The Stuff About Space That I Missed In 2014

2014 was a busy year for space exploration. Whilst I've covered a few of the notable stories and discoveries of the year, there have been many more that I've missed for one reason or another. To round off the year then, here is a brief look at four interesting space stories that I missed...

Three images showing the distant 2012 VP113 moving against a background of stars. Image Credit: Scott Sheppard/Carnegie Institution for Science
Whilst I was busy writing about the discovery of rings around the asteroid Chariklo, another discovery in the solar system was vying for attention. The image above shows three observations of the same patch of sky, each taken two hours after the last. Moving against the background of stars is a tiny, yet remarkable dot. 2012 VP113 is no more than a thousand kilometers across, but is currently over 80 Astronomical Units (AU, the distance between the Earth and the Sun) away from the Earth.

It won't get much closer, and will swing out to 240 AU from the Sun at the furthest point in its orbit. That puts it way beyond the orbit of Neptune, the furthest planet from the Sun. Only one other object, Sedna, is in a similar orbit to 2012 VP113. In fact their obits are suspiciously similar, leading the discoverers to suggest that their passage around the Sun could be being shaped by a unseen planet in the far reaches of the Solar System. Or it could be coincidence.  
Artists impression of WISE J085510.83-071442.5, the coldest known "star". Image Credit: Robert Hurt/JPL, Janella Williams/Penn State University
In April astronomers at Pennsylvania State University announced the discovery of a very strange object. WISE J085510.83-071442.5 is a brown dwarf, one of a mysterious, ill-defined class of object that bridge the gap between stars and planets. Using NASA's WISE and Spitzer space telescopes, the discoverers had found that this brown dwarf was not only one of the closest objects to the Sun outside of our Solar system, at just 7.2 light years, but was also the coldest brown dwarf known.

The temperature on WISE J085510.83-071442.5 is between -13 and -18 degrees Celsius, as cold as Earth's poles. The low temperature shows that the object has a mass of around 3 to 10 times that of Jupiter, which is actually a little small for a brown dwarf. In fact it doesn't really fit into any of our current categories of astronomical objects- making it even more interesting.

Lift off of the Soyuz rocket, beginning the ill-fated voyage of the Russian space sex geckos. Image Credit: Roscosmos
"Russian Sex Geckos Lost In Space", or variants thereof, must be in the running for best headline of all time. Last August the Russian space agency, Roscosmos, launched the latest in a series of Foton laboratories, uncrewed spacecraft based on the old Vostock capsules. On board this particular launch were experiments studying the effects of spaceflight on various plants and animals. They included five geckos, sent into space to so see how their behaviour and sexual activity responded to weightlessness.

Disaster struck a few days after launch, as ground controllers lost contact with the tiny spacecraft. Stranded in the wrong orbit, the geckos were at risk of a fiery death in the Earth's upper atmosphere. Contact was restored a few day later, and the mission cut short before anything else went wrong. Sadly, when the capsule was recovered at the beginning of September, all of the geckos were dead. The Foton's heating systems had malfunctioned, freezing the unfortunate space lizards. Hopefully they died having a good time...

Artist's impression of the Kepler spacecraft, along with the different patches of sky that will be the targets of the K2 mission. Image Credit: NASA Ames/JPL-Caltech/T Pyle
Closing out the year was the news that the resurrected Kepler spacecraft has spotted an exoplanet, the first confirmed discovery of its new mission. Launched in 2009, Kepler spent the first years of its life staring at a single patch of sky, looking for tiny dips in the light caused by planets passing in front of their stars.

Although highly successful, with over one thousand planet candidates spotted, this technique relied on the spacecraft being able to precisely control the direction in which it was pointing. Kepler achieved this using four gyroscopes, or reaction wheels. Unfortunately by the middle of 2013 two of the wheels had failed, leaving Kepler at the mercy of the buffeting solar wind.

With a continuation of its original mission impossible, engineers at NASA came up with an ingenious solution to allow the stricken spacecraft to carry on hunting for planets.  The pressure from the solar wind, that would otherwise push it of course, can actually be used to stabilize the spacecraft in certain directions. This new mission, dubbed K2, will see Kepler stare at several patches of sky, remaining at each one for around 80 days.

The first confirmation of a new planet found with this technique was announced on 17th December.  HIP 116454 b has a diameter around two and half times the size of the Earth, with just under 12 times Earth's mass. This probably means that it is a small gas giant, known as a mini-Neptune. Hopefully this will be the first of many planetary discoveries from Kepler's new mission,  

My favourite space image of the year: The Philae lander heads off into the dark towards Comet 67P. Image Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

And that's all I have time to write about! There has been plenty more space stores this year, from the much-hyped but ultimately inconclusive BICEP2 results, interesting findings on Mars by NASA's Curiosity rover and a host of ups and downs for commercial spaceflight. More stuff about space will certainly come in 2015.

New blogs will be posted on Twitter. Happy New Year!

Tuesday, 9 December 2014

NASA Sucsessfuly Tests The Orion Spacecraft- But It's Not Going To Mars

A Delta IV Heavy rocket blasts off from Cape Canaveral carrying NASA's Orion Spacecraft on its first, unmanned test flight.
Image Credit: NASA/Bill Ingalls
Warning: There may be opinions ahead...

On Friday the 5th of  December the world watched as the most powerful rocket in the world blasted off from Cape Canaveral, Florida. Atop the bright orange Delta IV Heavy was NASA's new human spacecraft, Orion, making its first flight.

Although there weren't actually any people on it- that won't happen until 2021 at the earliest. This was an uncrewed test flight only, looping around the Earth then boosting out to nearly 6000 km high.

Four hours after launch Orion hit the top of Earth's atmosphere traveling at 32000 kilometers per hour, 85% of the speed it would have had if it it had come back from the Moon. Protected by its heat shield, Orion parachuted down into the Pacific Ocean in a scene reminiscent of the Apollo program.   

The mission was a complete success, testing several of Orion's key systems such as the huge heat shield,  avionics and separation systems. It also looked spectacular, with the whole flight relayed live to Earth via camera on the spacecraft. Those of you who follow me on Twitter will know I was thoroughly enjoying it.

There was one bit I didn't like though. NASA have been promoting this launch as the first step on a "Journey to Mars", part of the agency's aim to land humans onto the Red Planet in the 2030s. But I don't think Orion will ever go to Mars. In fact, at the moment it doesn't look like it's going anywhere.  

Orion drifts down on parachutes: The future of space flight, or a step into the past? Image Credit: NASA
Orion first began development as part of the Constellation program. Announced by George W. Bush after the loss of the Columbia space shuttle in 2003, the plan was to replace the space shuttle with two rockets that would land astronauts on the Moon.  The huge Ares V would do the heavy lifting, carrying the lunar lander and propulsion systems into orbit. A smaller Ares I rocket would then launch carrying Orion, The two parts would then dock in orbit and head off to the Moon.

Constellation looked very good on paper, an Apollo style return to the Moon planned for the early 2020s. Unfortunately it never received enough funding to meet its goals, and it was eventually cancelled in 2010 after just one test flight of a half-finished Ares I.

Instead of a return to the Moon, NASA was ordered to set its sights on Mars. It would turn to commercial companies, such as SpaceX and Orbital Sciences,  to replace the role of the space shuttle in supplying cargo and crew to the International Space Station. This would free NASA up to focus solely on developing the technologies needed to take humans to Mars.

Unfortunately this didn't go down well with a number of US politicians, for whom the cancellation of Constellation would mean severe job losses in the Sates that they represented. After much debate Orion was back, this time to launch on a new rocket, the Space Launch System, cobbled together out of parts left over from the Space Shuttle Program.

But the destination remained Mars, and Orion simply isn't built to do that. It's far too small, no bigger than a large car on the inside. For a journey to Mars, which could take up to a year, a much larger spacecraft will be needed.

NASA have talked about a Deep Space Habitat, a larger spacecraft that would be assembled in orbit to make the journey to Mars- although this is yet to even make it on  to the drawing board. Orion would be used to ferry astronauts up to it, and to bring them home at the end of the voyage.

But in this case it's far too large and expensive, tasked with a job that would be much better suited to the cheaper, purpose built commercial crew ships such as the SpaceX Dragon and Boeing CST100.

Orion has found itself in the worst of both worlds, too small to make the whole journey to Mars and unnecessarily big as a crew transport. And no wonder, as it's perfect for what it was designed to do: Go to the Moon. 

Worse still is its projected time table. Orion is so underfunded that the next test flight isn't until 2018, the first time that the Space Launch System will be ready. And it still wont be carrying any people- the first piloted flight is planned for no earlier than 2021.

NASA doesn't have enough money to build the life support systems yet, so the test flight last week couldn't have carried people even if they'd wanted it to. With up to two new US Presidents between it and its first crewed flight, Orion's chances of ever flying with humans on board are shaky at best.

This isn't to say that we wont go to Mars, or that we can't. I think we should, and will have the technological capability to do so within my lifetime.

But I doubt that Orion will be a part of it.

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Friday, 21 November 2014

The First Landing on a Comet

Released from the Rosetta orbiter, the fridge-sized Philae lander drifts down to become the first spacecraft to land on a comet. Image Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA 

And the second, and the third...

At 8.35 GMT last Wednesday morning, five hundred million kilometres from the Earth, a tiny lander called Philae detached from the side of the Rosetta spacecraft. 28 minutes later the signal confirming the separation arrived at ESA’s Space Operation Centre (ESOC) in Darmstadt, Germany. The first ever attempt to land a spacecraft on a comet had begun.

Unlike most spacecraft landings, Philae would not land using rocket engines or parachutes. Rosetta had pushed it away in (it was hoped) just the right direction, at just the right speed to fall gently down onto its target.

The target was Comet 67P/Churyumov–Gerasimenko, an irregular lump of dust and ice less than five kilometres across at its widest point. Separating from Rosetta 22.5 kilometres from the surface, the low gravity of Comet 67P pulled Philae into a leisurely, seven-hour descent. 

As it fell towards Comet 67P, Philae had time to spin round and take a picture of Rosetta...
Image Credit: ESA/Rosetta/Philae/CIVA 
...whilst Rosetta watched Philae disappear into the darkness.
Imaged during its descent by Rosetta's OSIRIS camera in the sequence above, Philae is a 100kg box filled with ten scientific instruments, including cameras, spectrometers, a drill and two labs for analysing surface samples. And, crucially, two harpoons.

These harpoons were to fire as Philae touched down onto the surface of the comet, anchoring itself securely to 67P. The plan had been for a small thruster on the top of the lander to ignite at the same time, holding Philae down onto the surface. But that morning, the team at mission control had discovered that the thruster had stopped working. Only the harpoons could stop Philae from rebounding off the surface of Churyumov–Gerasimenko and back into space.

A picture of the first landing site from 40 meters above the surface. Image Credit: ESA/Rosetta/Philae/ROLIS/DLR
At this point I had to go to a seminar, and spent the next tow hours failing to pay attention to the speaker whilst surreptitiously checking Twitter for news. If Tom Shanks is reading this, then sorry! But I got out in time to celebrate with the rest of the world as, at 16.03 GMT, the signal arrived at ESOC: Philae had landed, the first spacecraft to touch down on a comet. There was much rejoicing.

But the celebrations were short lived. As the mission controllers studied the data relayed back by the orbiting Rosetta, they realised that the crucial harpoons had failed to deploy. Worse still, the signal from the lander was fading in and out, and the power being generated by its solar panels was varying wildly. By the evening, a tentative explanation had been found: Philae had bounced straight off the comet and gone into a spin.

In a series of incredibly detailed images, the orbiting Rosetta spacecraft tracks Philae's wild flight across the surface of Comet 67P. Image Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
By the next morning the full tale of the landing had been put together. Philae had landed at 15:34 GMT, thudding down in exactly the right place. But without the thruster or harpoons to hold it down, the tiny spacecraft had bounced back up again, heading off the comet at a leisurely 38cm/s. Thanks to the extremely low gravity of 67P, Philae flew over the surface for nearly two hours, flying almost a kilometre high. During that time the comet turned underneath it, the targeted landing site slipping away.

When Philae hit the ground again it made a second bounce, this time for only seven minutes. When it finally came to a halt, the lander was over a kilometre from the spot where it had first touched down. But exactly where Philae had ended up was a mystery.  
Panoramic view of Philae's landing site, with the spacecraft superimposed. It wasn't meant to be this dark... Image Credit: ESA/Rosetta/Philae/CIVA
The first images from the landing site showed a very different place to the flat, sunny target. Philae appeared to be at a tilt, with one leg sicking into space. Worse still, the bulk of the lander's solar panels were in the shadow of a large cliff. If Philae wasn't able to move, then it would only get around 1.5 hours of sunlight each day- nowhere near enough to recharge it's batteries.

But Philae was designed with this scenario in mind. Although the solar panels would have allowed it to carry on working for several months, it had been built with enough battery power to complete all of its initial science observations. While the mission controllers pondered a way to move away from the cliff, Philae's ten instruments swung into action.

The ten scientific instruments Philae used to study the surface of Churyumov–Gerasimenko.  Image Credit: ESA/ATG medialab

The full results from the measurements made by Philae have yet to be released, but a few preliminary discoveries have been announced. Particularly intriguing was the data collected by the Multi-Purpose Sensors for Surface and Subsurface Science, or MUPUS. This instrument deployed a small hammer, deigned to dig into the surface of Churyumov–Gerasimenko and measure the temperature at different depths.

Surprisingly, even at it's most powerful setting, the hammer couldn't make a dent in the surface of 67P. The ground beneath Philae, long expected to be a porous, loosely bound mix of dust and ice, was actually rock-solid. Although this conflicted with accepted knowledge (always a good kind measurement to make), a solid ice crust would explain why Philae bounced so high after its first touchdown. The low density of the comet suggests that, beneath this icy crust, the material of Comet 67P is much less tightly packed.

Another instrument, the Cometary Sampling and Composition Experiment or COSAC, detected signs of organic chemical compounds on the surface of Comet 67P. These carbon-rich compounds, which give the comet its deep black colour, are one of the key reasons we are interested in these icy worlds. It is thought that many of the ingredients needed for life on Earth, such as water and some amino acids, were originally delivered here by impacting comets.

With battery power running low, Philae ran through all of it's remaining scientific instruments, drilling into the surface to collect material for its onboard laboratories, receiving and transmitting radar data from Rosetta to map the insides of the comet, and taking yet more images.

By the time its batteries finally gave out, Philae had achieved all of its planned science operations. Despite the bumpy landing, the mission had been a complete success.  
At 36 minutes past midnight on Saturday morning, mission control at ESOC lost contact with Philae. But there's still hope for the little lander. Just before its batteries gave out, Philae had managed to turn itself, bringing it's largest solar panel out of the shade into the faint sunlight.

As Churyumov–Gerasimenko flies ever closer to the Sun, there's a small chance that Philae's batteries will recharge. We may yet be hearing more from the tiny lander. Even if this is the end of Philae's epic adventure, Rosetta is still in orbit of the comet, continuing to revolutionize our knowledge of these tiny, mysterious worlds.

Note: you my have noticed that I haven't commented on #shirtstorm- it's outside the scope of what I wanted (and feel qualified) to talk about, but I recommend and broadly agree with articles like these on the issue.

Another Note: I've stared writing for Astrobites! These are daily summaries  of recent scientific papers, written by astronomy postgraduate students. The style is a bit more technical than this blog, but it's worth a look if you want to to keep up to date with astronomy research. I'll be writing there once a month, and my first post is here

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Tuesday, 11 November 2014

How to Watch the Comet Landing!

On the 12th of November the European Space Agency will make the first ever attempt to land a space probe on a comet. If all goes to plan, the Rosetta orbiter will deploy the Philae lander into a seven-hour drift onto the surface of Comet 67P  Churyumov–Gerasimenko.  I'll do a full blog after the event, but here's some hopefully useful bits and pieces to follow the high point of the Rosetta mission:

The first port of call is the ESA Livestream, where all the major events will be shown and new data announced. If I've made this work right, it should be playing above this paragraph. It can also be found (along with lots of other stuff) on the Rosetta homepage.

Timeline of Philae's seven-hour descent onto Comet 67P. The signal from a successful touchdown should arrive at or after 4pm GMT. Click on the image to enlarge. Image Credit: ESA 
Above are the key points in the landing sequence. A much more detailed version is available here.

Image of Philae's targeted landing site, known as Agilkia. Image Credit: ESA
A bit of a wider view: The image above shows the target landing site on the "head" of Comet 67P. It's been named  Agilka, after an island in the River Nile where the temple from the island of Philae was moved to avoid flooding caused by the building of a dam. Philae was the place where the Rosetta Stone was found.

Apart from the ESA Livestream, the best place to stay up to date with the landing is proably Twitter. I will be tweeting updates and my feed should hopefully be appearing below this paragraph. You should also have a look at #CometLanding.

Good luck Philae!

Friday, 7 November 2014

ALMA Spots Planets Forming Around a Young Star

High-resolution image of the protoplanetary disc around HL Tau, a young star roughly 450 light-years from Earth. The image, which was taken by the ALMA telescope, shows gaps and rings in the disc carved out by new-born planets. Image Credit: ALMA (ESO/NAOJ/NRAO) 
This morning the image at the top of this page was doing the rounds on Twitter. I, like several others, glanced at it and initially moved on. I've seen plenty of artist's impressions like it before. It took me a while to realise that this isn't a painting. This is a real image from the ALMA telescope, showing the birth of a solar system.

The image shows a star surrounded by a protoplanetary disc, a huge ring of gas and dust around twice the diameter of Neptune's orbit. Invisible at the wavelengths of light that ALMA sees, the central star is a young object called HL Tau, which is  just a million years old. That might seem old, but our own Sun, which is otherwise quite similar to HL Tau, is 4.6 billion years old. HL Tau is a star at the very beginning of its life.

This makes the disc partly expected, but partly mysterious. For the past few decades most models of how planets form have been based on discs like these, the leftover debris from the cloud that collapsed to form the star.  HL Tau is making planets.

Although the entire process is till not fully understood, the theory suggests that slight irregularities in the disc can cause some areas to become more dense. This makes them clump together, growing from dust into small rocks. As they get larger their gravity gets stronger, pulling in more and more material until they begin to look like small planets or asteroids. These planetesimals begin to collide, combining to eventually form planets.  

The ALMA image is a resounding confirmation of this theory, showing this process in action. The disc has huge gaps in it, gaps which are carved out by newly-forming planets. This is the mysterious part, as the presence of these very well defined gaps, at such a young star, show that the planets must be growing much quicker than many simulations suggest.

Not all of the gaps will have planets in them. Some of them will be formed by resonances. This means that, for example, an area of the disc could be going round the star a certain, precise  number of times in the time it takes a further out planet to go round once.

For example, a dust particle in the disc could be going round the star four times for every time planet, which is further away form the star, goes round once. The planet and the dust will then be lined up at exactly the same place each time the planet goes around the star. The gravity of the planet will give the dust an identical tug or kick each time, moving it out of it's orbit.

As this will happen to all of the dust in the same resonant orbit, eventually a gap is cleared. We see the same behaviour in this Solar System- the many rings of Saturn are shaped and sculpted by moons in resonant orbits. Which of the rings in HL Tau are formed by planets, and which are cleared out orbital resonances, will take more observations to find. The full research paper on this observation is yet to be published, so maybe we'll find out then.

Hubble Space Telescope image of the clouds of star-forming gas and dust around HL Tau. Image Credit: ALMA (ESO/NAOJ/NRAO)/NASA/ESA
HL Tau is in the constellation Taurus, currently visible in the late evening in the Eastern sky, near the Moon. But you wont spot the disc, it's far too small.

This leads to the second incredible part of this image: The resolution. The image was taken by the Atacama Large Millimeter/submillimeter Array (ALMA), a huge array of 43 (and counting) telescopes designed observing in the submilliter wavelength range, between infrared and radio waves. The telescopes are spread out by up to fifteen kilometers, allowing them to take extremely sharp images.

ALMA can distinguish between objects separated on the sky by just 35 milliarcseconds. For comparison, your eye has a resolution of around one arcminute- nearly two thousand times worse. Resolution like ALMA would allow you to see both sides of a penny placed over one hundred kilometers away.

It's this high resolution that has allowed ALMA to see the protoplanetary disc in such exquisite detail, picking out the tracks formed by the new, growing planets. And this is in many ways just a proof of concept, a test of ALMA's capabilities. Hopefully we'll be seeing many more amazing things with this telescope in the future.

New blogs will be posted, as ever, on Twitter.

COMING SOON: Next Wednesday (12th) The Rosetta spacecraft will deploy the Philae lander to make the first attempt to land on a comet. I'll be tweeting and blogging along, and I highly recommend keeping track of, in my opinion, the most exciting space event of the year.

Friday, 24 October 2014

Hubble spots Comet Siding Spring flying past Mars

Comet Siding Spring makes near miss of Mars in this image form the Hubble Space Telescope. Click to enlarge! Image Credit: NASA, ESA, PSI, JHU/APL, STScI/AURA 
Last Sunday the comet C/2013 A1  Siding Spring flew past Mars at a distance of just 140 thousand kilometers, or one third of the distance between the Earth and the Moon. It's the closest we've ever seen a comet get to a rocky planet- so close, in fact, that initial observations suggested it might even hit.

Data and observations are pouring in from the flotilla of orbiters around the Red Planet, and I'll certainly write more on this story as more results become available. For this blog post though I just want to show this amazing image, taken by the Hubble Space Telescope, showing Siding Spring at its closest point to Mars. Click on the image to zoom in.

This photo, which is defiantly now on my list of favourite space images, is actually a composite of two observations, one of Siding Spring and another of Mars. Although both objects would have fitted in the field of view of Wide Field Camera 3, the instrument used to obtain the image, Mars is around ten thousand times brighter than the comet. An exposure long enough to see any detail in Siding Spring would have captured Mars as just a shining white blob! A second problem that Siding Spring was moving across the sky much faster than Mars. Hubble had to track across the sky in time with it's motion, so a picture of Mars taken at the same time would have been a blur.

Photographic trickery aside, the result is incredible. I especially like the amount of detail on Mars, as well as the structure visible in Siding Spring's tail.

Much more on Siding Spring to come! Followed by the the main comet-related event of the year on 12th November, when the Rosetta spacecraft will send down a lander to make the first attempt at landing on a comet.

As always, follow me on Twitter for more stuff about space.

Friday, 3 October 2014

Two new spacecraft join the Mars flotilla

The best view of Mars form Earth, taken with the Hubble Space Telescope. A growing number of spacecraft have been sent to study the Red Planet from close-up, including MAVEN and MOM ,which arrived this week. Image Credit: NASA/ESA and The Hubble Heritage Team STScI/AURA
On the 14th of July 1965, Mariner 4 became the first spacecraft to successfully flyby Mars, providing the first close-up images of the fourth planet from the Sun. Since then a host of spacecraft from several nations and space agencys  have flown past, orbited or even landed on Mars (along with many, many failures).

Growing interest in an eventual human mission to Mars has seen a surge in such missions over the past few years, most of them successful. Last week two new spacecraft joined the international flotilla of orbiters and rovers, including India's first interplanetary mission. So here, in order of arrival, are all of the active missions and what they're teaching us about Mars

Mars Odyssey

Artist's impression of Mars Odyssey, the oldest active spacecraft at Mars. Image Credit: NASA/JPL-Caltech
The first spacecraft to arrive at Mars in the twenty-first century was Mars Odyssey. Named after the book (and film) 2001: A Space Odyssey, this NASA orbiter reached Mars in, appropriately enough, 2001. Designed to study the chemistry of the Martian surface, its key discovery was the detection in 2002 of vast amounts of water ice lying just below the ground (click that link to be amazed by 12 year old internet...)

Whilst it has continued to make scientific observations, Odyssey has in more recent years fulfilled a vital role as a communications relay, transmitting information from the various landers and rovers on the surface to Earth and relaying commands back. I quite like this fact- we're beginning to build a space-based communications infrastructure at another planet!

Mars Express

Mars Express, the first European Mars orbiter. The long booms form the MARSIS sub-surface sounding radar, used to map the geology of the top few kilometres of the Martian crust. Image Credit: NASA  
Arriving in December 2003, Mars Express was the first European Space Agency mission to another planet. Based on the design of my current favourite mission, Rosetta, and sister craft to the near-identical Venus Express, Mars Express carries instruments to measure the chemical composition of the Martian atmosphere, surface and even subsurface. It also has a nifty spectroscopic camera allowing it to take high resolution, 3D images of Mars and, thanks to its unusually elliptical orbit, Mars' largest moon, Phobos.

In 2004 the spectrometers on Mars Express made an intriguing observation: signs of what could have been methane in the atmosphere. Methane should only last a few hundred years in an atmosphere before it reacts with the other chemicals around it, so for it to be present in detectable amounts means that something must be producing it. We know of several geological processes that could achieve this, but most methane production on Earth is biological. Could Mars Express have seen signs of life?

Mars Express also carried a lander, the British-built Beagle 2. Sadly however the landing was a failure, and contact was lost with Beagle 2 shortly after it entered the atmosphere on Christmas Day 2003. The reason for its loss is still unknown.


Panorama of Endurance Crater taken by the Opportunity Rover in 2004. One of the rover's solar panels can be seen in the bottom right (click to make bigger). Image Credit: NASA/JPL/Cornell 
The Mars Exploration Rover Opportunity is currently over ten years into a 90 day mission. Yep, you read that right.

January 2004 saw the arrival of two identical, six wheeled rovers on Mars. Following on from the highly successful Pathfinder mission, Spirit and Opportunity parachuted through the thin atmosphere and landed via an innovative airbag system. Original planned to last just three months and drive around a kilometre across the surface, both rovers far exceeded their targets. Spirit became stuck in sand in 2009 and didn't survive the winter (the xkcd on the topic is essential reading), but Opportunity is still going strong, having covered a distance of over 40 kilometres.

The full list of discoveries made by this stupendously successful mission would be several posts on its own, so in the interests of word count I'll talk about just one. Early in its mission Opportunity was sent to investigate the wreckage of the heat shield that had protected it during its entry into the Martian atmosphere. Near the heat shield was a strange, dark-coloured rock, out of place with the geology around it. Opportunity had discovered the first meteorite on anther planet.

The meteorite, dubbed Heat Shield Rock, was a lump of iron and nickel leas than half a metre across. Its existence was a mystery: Mars' thin atmosphere couldn't have slowed it down enough to stop it vaporising when it hit the ground. So perhaps at some point in the past Mars had a much thicker atmosphere, an atmosphere that it has since lost?

Mars Reconnaissance Orbiter
Over 250km above the Martian surface, the Mars Reconnaissance Orbiter easily spots the 1.6 metre long Opportunity. Image Credit: NASA
Since the before the start of the Space Age, landing people on Mars been high up the wish-list of things to do in space. A key requirement for that, as well as for larger robotic landers, is high-resolution mapping of the Martian surface. In 2006 the Mars Reconnaissance Orbiter (MRO) arrived to do just that.

Significantly larger than its predecessors, MRO's main instrument is 0.5 metre downwards-pointing telescope. The High Resolution Imaging Science Experiment, or HiRISE, is the largest telescope ever sent to another planet and can image the Martian surface at resolutions down almost 30sm/pixel.

HiRISE, together with several other instruments, has allowed us to explore huge swaths of the Martian surface in great detail. Among its many achievements has been providing evidence for brief flows of running water, as well as spotting parachuting landers heading down to the surface.


The largest lander ever sent to another planet, the Mars Science Laboratory Curiosity landed via a highly complex skycrane system in 2012. Here, the nuclear-powered rover takes a selfie, next to a rock that it has drilled into  (middle left) to obtain a sample for it's onboard laboratory. Image Credit: NASA 
A common complaint about a perceived lack of technological progress is "where's my jetpack?" Whilst a person using a jetpack would actually be a really silly idea, that question does now have an answer. It's on a nuclear powered, laser equipped mobile science lab on Mars.

At 900kg, the Mars Science Laboratory, better known as Curiosity, was far too large for it to land using air bags like Spirit and Opportunity. Instead they used a skycrane, a rocket powered aircraft that slowed the rover down from 200 mph to zero before lowering it down on cables. This video has the full details of an operation that surely ranks among  the most difficult and technologically impressive achievements of humankind.

Curiosity's primary mission on Mars was to determine if the conditions on Mars could at some point in its past have been suitable for life. By the end of its first (Earth) year on the Red Planet Curiosity had met its scientific objectives, showing that the rocks around it had once formed part of a lake bed, with water and all of the chemical ingredients needed for life.

The Mars of several billion years ago was evidently very different to the barren planet we see today. However, Curiosity found no trace of the methane in the atmosphere that had been detected years earlier by Mars Express.      

Completing its primary mission in August, it has not all been smooth driving for Curiosity. NASA's recent Senior Review of its planetary exploration missions found that the rover was not being used to its full scientific potential, and that a better balance between  driving and taking data needs to be found. Whatever its troubles, Curiosity will certainly make more exciting discoveries over the next few years, as it begins to climb a 6km high mountain.


Arriving at Mars last week, MAVEN has been sent to find out what happened to Mars' atmosphere. Image Credit: NASA
Finally we come to last week's new arrivals. First to arrive was the  Mars Atmosphere and Volatile Evolution (MAVEN), a NASA mission. As a wide range of general scientific capabilities is already present at Mars, MAVEN's mission is somewhat more specialized than previous spacecraft. Its primary objective is to find out what happened to Mars atmosphere.

To support, for example, the prehistoric running water and intact meteorites found by previous missions, Mars must have had a thick atmosphere similar to the Earth's. Yet all that remains now is thin shell of carbon dioxide. Where did the atmosphere go?

In an attempt to answer these questions MAVEN will be sent on a daring mission into the upper reaches of Mars' atmosphere. The bent shape of its solar panels, seen in the artists impression above, will help with this, allowing it to remain stable as it becomes in effect our first interplanetary aircraft.

There, its advanced suite of spectrometers along with a magnetometer, will measure in detail the composition of the atmosphere and, crucially, its interaction with the solar wind. As Mars has no global magnetic field to protect it, the force of the solar wind has become the prime suspect in the case of the missing atmosphere.


MOM is the only Mars orbiter capable of taking images of the whole of Mars in one go. Compare with the HST image at the start! Image Credit: ISRO
Arriving into Martian orbit on Friday 24th September, The Indian Space Research Organisation's Mars Orbiter Mission (MOM) has one notable difference with all the spacecraft to have come before it: The price tag.

Although the quote value of $74 million probably doesn't take all of its costs into account, MOM still cost many times less than MAVEN. Despite this, it has a small yet advanced suite of instruments, These include the first camera capable of taking full-disc images of Mars, as well as a dedicated methane detector which will try and finally nail down the story of this elusive gas in Mars' atmosphere. Although MOM only has a six-month mission planned, plans made this week to share science data with NASA suggest that it many well keep going for some time.

India's success in pacing a spacecraft into Martin orbit may be the start of a new stage in space exploration, showing that exploring the solar system isn't limited to a few select countries and can be done without spending billions. India, and the countries that follow it, will reap the technological benefits from these missions just as the "traditional" spacefarers have done before.

And that's it! With seven working spacecraft now on or orbiting Mars, as well as more missions launching soon, we are learning more about the fourth planet from the Sun than ever before. Within our lifetimes we may finally find out whether Mars once supported life, or even if it has any now. And the more we study the Red Planet and how to get there, the closer humankind gets to finally voyaging from Earth to join our robotic explorers.

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Wednesday, 10 September 2014

Blink and You'll Miss it: The Brief Life of a Gasous Disc around a White Dwarf

Artist's impression of a debris disc in orbit around a white dwarf. Over the past eight years we have seen such a disc suddenly form, then rapidly disappear around the white dwarf SDSS J1617+1620. Image Credit: NASA, ESA, STScI, and G. Bacon
 Today I published my first research paper, graced with the snappy name of "Variable Emission from a Gaseous Disc around a Metal-Polluted White Dwarf". The tone of the title is a hint to the tone of the paper, so I'll attempt to provide a much more readable post about what we've done here.

(Note: Where I use "we " below I'm generally referring to all of the authors of the paper. I didn't actually start working on this project until January, so other people have done a lot of work on this before me.)

History of a dying star

Around four hundred million years ago, SDSS J1670+1620 was a star similar to our Sun (although with 2-3 times the mass). As it reached the end of the hydrogen fuel supply in its core, it swelled up into a red giant, then blew away its outer layers to become the tiny white dwarf that we see today.

Just like the Sun, SDSS J1670+1620 hosted a system of planets, asteroids and other objects. As the star underwent the turbulent transformation into a white dwarf its inner planets were destroyed, tumbling into the red giant.

The rest of the system may have survived relatively unscathed. The remaining planets would have started moving out into wider orbits as SDSS J1617+1620 shrank down to a tiny ball of carbon and oxygen the size of the Earth, with only around a third of its original mass.

Systems like SDSS J1617+1620 are referred to as evolved planetary systems. Over the past couple of decades astronomers have discovered dozens of systems like this, and we now know that they exist at around at least half of all white dwarfs.

Around 33 (and counting) white dwarfs we've detected rings of dust. These structures, as wide across as the Sun but only a few millimetres thick, are formed by asteroids thrown in towards the white dwarf by distant planets. When they get too close the gravity of the dead star overcomes the forces bonding the rock of the asteroids together, ripping them apart into a dusty disc.

Such a disc was observed around SDSS J1617+1620 in 2012. But as well as a dust disc, we also saw signs of a much rarer phenomenon: Another disc, this one made of gas.

How to find a gas disc

The way we find these gaseous discs is, in my opinion, rather cool- another one of those places where a seemingly complicated plot clearly shows what's actually there.
"Diagram" showing how we observe gas discs. Instead of a single colour in the spectrum, the light emitted from calcium is spread out into a double peak. The red- and blue-shifted light is from the parts of the disc moving away from and towards us respectively. Image Credit: Me.
The diagram above, as well as demonstrating my amazing Paint skills, shows how we find the gas discs. First the light from the white dwarf is split into a spectrum separating it out into all the different colours coming from the Star. A rainbow is the result of this happening to the Sun; in effect we are making a star rainbow.

Each element in the object emitting the light leaves its mark on the spectrum, emitting or absorbing light at very specific wavelengths. When we look at the emission from calcium, we see something odd. Instead of a sharp line, the emission is smeared out into as distinctive double-peak, with one peak on each side of where the line should be.

This is a result of the Doppler Effect. If an object emitting light is moving towards us, the light waves get bunched up, resulting in a lower wavelength. The light appears to be slightly more blue than expected. As it moves away, the light is stretched out and the wavelength increases, becoming more red. The Doppler Effect has many applications, most famously being used by Edwin Hubble to show that the universe is expanding.  

With this in mind we can see that the double-peak emission must be coming from a disc around the white dwarf. As the disc rotates, the side of it moving away from us emits red-shifted light, and the side moving towards us emits blue-shifted light.

Not only can we tell that there's a disc, but we can also measure its size and location. The amount that the light is shifted by depends on the speed of the material in the disc, which depends in turn on how far away the gas is from the white dwarf. Measuring the wavelengths of the inner and outer extent of the peaks can therefore tell us the exact position of the disc.

Many mysteries

Doing this measurement has had a surprising result. Including this one, we've seen gaseous discs around just seven white dwarfs. They should have been easy to explain: As the dust gets closer into the white dwarf, the heat should sublimate it into gas.

But when the positions of the gas discs were measured, they were all found to be too far away from the white dwarf for this to happen. The question is then: if it's not the dust sublimating, how did they form? And why do we see them at only a small number of white dwarfs?

A key missing piece of evidence is variability. As the discs are formed by asteroids, which are regularly scattered in towards the white dwarf, we know that these are highly active systems. But despite being observed over several years or even decades, none of the gas or dust discs have shown any changes with time.

Until now.

Slide show of some of the spectra of SDSS J1617+1620. In the space of two years double-peaked emission lines revealing a gaseous disc appeared out of nowhere. A couple of years later the emission had decreased dramatically, and by last year the disc had completely disappeared.

The disappearing disc

After discovering the first gaseous disc around a white dwarf in 2006, a search was made in the Sloan Digital Sky Survey for more white dwarfs with double-peaked calcium emission lines.

Two observations had been made of  SDSS J1617+1620 one in 2006 and later in 2008. The later spectrum showed clear evidence for a gaseous disc, a huge structure as wide across as the Sun. However, in the earlier spectrum the disc was nowhere to be seen.

Intrigued by this clear, unprecedented evidence for variability, we started observing the white dwarf with ever more powerful telescopes. To our surprise, the gaseous disc dispersed almost as quickly as it has formed. Just a year after the observations first showing the disc the emission from the disc had fallen by more than half. The gas disc continued to dissipate, until by 2013 there was nothing left.

How did this happen? The video above shows one possibility. We know that SDSS J1617+1620 is orbited by a dust debris disc, formed form the shattered remains of an asteroid. A second asteroid, thrown in from the outer reaches of the system by an orbiting planet, could have impacted in this disc, creating a short-lived burst of gas.

A more intriguing possibility is that we're actually seeing the formation of the dust disc itself. Depending on how close it gets to the white dwarf, an asteroid being pulled apart by gravity might not turn entirely to dust in one go. It might take several orbits, passing close to the white dwarf and loosing a little more mass to the forming disc each time before moving away again.

Each time the debris of the doomed asteroid returned it would interact with the growing dust disc, forming a gaseous layer like the one we're seen. The intriguing possibility here is that the asteroid may return again in a few years time, perhaps creating another gas disc. This could provide a key insight into the dynamics of evolved planetary systems, and we plan to keep observing this star to find out if the gas disc returns.

However it formed, we also need to try and explain how the gas disc disappeared so quickly. Here we have a helping hand, as there has been plenty of theory work done studying the behaviour of gaseous and dusty discs.

Each particle of dust orbits the white dwarf independently, interacting very rarely with the rest of the dust in the disc. The gas, however, behaves more like one big object, with each particle mixing and colliding with those around it. This has the effect of slowing the gas (and dust) down, causing it to spiral inwards and fall onto the white dwarf. The more gas, the faster the accretion.

This accretion is easily visible as metal-pollution in the otherwise pure hydrogen atmosphere of SDSS J1617+1620. We should be able to test this theory, seeing if the accretion rate changes in time with the variability of the gas disc

However when we measured the accretion rates, they stayed the same. This could mean that our ideas about the behaviour of the gas discs are wrong, but its more likely that there was too little gas to cause a noticeable effect- this would also explain the short lifetime of the disc. Alternatively there could be a delay between the formation of the disc and the change in accretion rate as the disturbed material moves inward. More reason the keep watching!

The future

 As we were finishing off this paper, another paper was published showing variability at another white dwarf- but this time it was the dusty part of the disc changing, not the gas. With clear evidence that both types of disc at white dwarfs can change, we may be able to begin to answer some of the outstanding questions about these intriguing systems.

Studying these systems also provides an insight into the future of our own Solar System. Five billion years from now, the Sun will begin the transformation into a white dwarf. The more we learn about evolved planetary systems, the better we'll understand what happens next.

The full paper is available here. I haven't gone into all of the details about what we've discovered at SDSS J1617+1620 as that would be a really long post, but if you want to find out more feel free to ask me any questions in the comments or on Twitter.

Thursday, 7 August 2014

Rosetta Enters orbit around Comet 67P/Churyumov-Gerasimenko

A strange new world: Comet 67P/Churyumov-Gerasimenko as seen by the Rosetta spacecraft, which became the first spacecraft to orbit a comet on 6th August. Image Credit: ESA.
After a 10 year voyage through the the solar system, the European Space Agency's robotic explorer Rosetta has become the first spacecraft to enter orbit around a comet. Since waking up in January from a two-and-a-half year hibernation, Rosetta has been steadily gaining on on comet Churyumov-Gerasimenko, also known as 67P. On Wednesday morning it finally arrived, burning its main engine for 6 minutes and 26 seconds to reach a relative speed with Comet 67P of just one meter per second.

The surface of Comet 67P/Churyumov-Gerasimenko taken from a distance of 100km with Rosetta's OSIRIS science camera. Image Credit: ESA
A few hours later Rosetta returned the first close up images from the comet, our first good look at a completely new world since the Dawn spacecraft orbited Vesta in 2011. The image above, with a scale of around 2.5 meters per pixel, reveals a varied topography strewn with boulders.

Previous missions and telescope observations have revealed that comets like Churyumov-Gerasimenko are "dirty snowballs", irregular mixes of ice and dust. Working out how that chemical composition and the varying geological activity of the comet has produced such a landscape is one of the questions that Rosetta will try and answer.       

Video: ESA

Unlike most space missions, Rosetta's initial orbit around Comet 67P doesn't follow the standard circle or ellipse. Until the mass of the comet can be measured by observing its gravitational pull on Rosetta, the ground controllers at ESA don't know exactly what manoeuvres will be needed to reach a stable orbit.

Instead, as the video shows, Rosetta will fly around the comet in a strange triangular orbit, flying in hyperbolic arcs with thruster burns at each corner. From there the orbit will be slowly lowered, until the spacecraft is in an ellipse just 10km above the surface of Comet 67P.

An overexposed image of Comet 67P taken on 2nd August, revealing jets of material streaming from the surface. Image Credit: ESA 
The seeming tranquillity of the first close-up images is deceptive. I've already written about how Rosetta has seen the activity of Churyumov-Gerasimenko increase as it gets closer to the Sun, and this overexposed image shows two distinct plumes of material streaming out of the surface of the comet.

This activity will continue to increase during Rosetta's time at the comet. By the time Churyumov-Gerasimenko reaches perihelion, the closest point in its orbit to the Sun in a year's time,  the plumes will have grown into a characteristic tail, or coma.

Video: DLR

Arguably the most exciting phase of the mission is still to come. In November Rosetta will deploy the Philae lander, a fridge-sized box that will attempt to become the first man-made object to land on a comet. Rosetta has already made an initial search of Churyumov-Gerasimenko for possible landing sites, shown as green cricles in the video. Over the next few months this will be narrowed down  to one area for Philae to target, guiding itself in with a pair of harpoons.

With a successful orbital insertion, the Rosetta mission is shaping up to be one of the most exciting space missions ever carried out. The pictures and data that it is returning are already fantastic, and I'm sure I'll write about it again as the mission continues.

P.S. Last time a wrote about Rosetta, I was contacted by a group working on a website where you can see a visualisation of the whole mission, charting the entire ten-year voyage up until now. I recommend a look.

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Tuesday, 5 August 2014

Planets Across the HR Diagram Part 1

I've just got back from my first conference in Cambridge, "Characterizing Planetary Systems Across the HR Diagram". The conference attempted to provide an opportunity for people working on planets in different ways, like trying to find out how they formed, how they behave, what they look like and a host of other questions, to try and join up some of the gaps between their areas.

Over the next couple of posts I'll try and record some of the bits of the many talks and presentations I found the most interesting, but I should probably start by explaining the name of the conference.

A simplified Hertzsprung-Russel (HR) Diagram, a plot of all of the stars in the sky. The horizontal axis shows a star's surface temperature. The vertical axis shows how bright the star is.  Image Credit: ESO.
The Hertzprung-Russel, or HR, diagram is a plot of all of the stars on the sky according to how bright they are and what temperature they are. A star's position on the diagram can tell us a huge amount about what it is like and at what stage in its life it is.

Along the horizontal axis is the colour that the light from the star appears as, going from blue through yellow to red. Stars give of light in a particular way known as a blackbody spectrum, a consequence of which is that the colour that a star appears as is directly linked (with a few other things taken into account) to its surface temperature. This means that the horizontal axis is also a measure of a stars temperature. As shown in the picture, the temperature (confusingly) increases from right to left.

The vertical axis shows how bright a star is, the stars luminosity or magnitude. Note that this isn't a measurement of how bright the star appears to be from Earth (the apparent magnitude). This doesn't tell us what the star is like, as a dim, nearby star can have the same apparent magnitude as a bright star further away. The HR diagram measures how bright a star actually is, defined either by its magnitude at a set distance (the absolute magnitude) or by how much energy it gives out (its luminosity). The HR diagram shows here measures the luminosity of the stars as compared to the luminosity of the Sun.

The position of a star on the HR diagram is related to what stage it is in its life-cycle. Stars spend most of their lives on the wavy line going from bottom right to top left across the diagram. This is known as the Main-Sequence (MS), and it's where the Sun is now. Less massive, redder stars like red dwarfs are towards the bottom right of the MS, whilst massive, hot blue stars are near the top left.

At the end of their lives, most stars swell up into red giant stars many times bigger than the Sun. Whilst the temperature of the star doesn't change that much, the surface area and hence the luminosity of the star will increase dramatically. They therefore move up the HR diagram into the top right.      

From here around 5% of stars explode as supernovae. The rest blow off their outer layers, leaving behind a tiny, very dim but very hot white dwarf. This moves them to the bottom of the HR diagram.

All of this means that by plotting a star on the HR diagram we can immediately tell what kind of star it is, and at what stage it is in its life. This has many applications. For example we can use it to tell how old star clusters are, looking to see if the massive, shorter-lived stars in the top left of the HR diagram are missing.

So that's the HR diagram part of the conference name explained. Now for the "Characterizing Planetary Systems..." part.

In his talk on the first day, Kevin Schlaufman showed us a different version of the HR diagram:

Kevin Schlaufmann's image of the known exoplanet-hosting stars on the HR diagram. Image Credit: Schlaufmann et al 2013. 
This HR diagram shows only those stars that have been confirmed to host exoplanets. The most notable feature is how empty it is compared to the full HR diagram, showing the huge gaps in our knowledge about planets around giants, white dwarfs and the top end of the main sequence.

This is partly due to the techniques we use to search for exoplanets, which tend to be biased towards finding planets at smaller stars. But it also revels how little we know about planets in some of these areas.

The aim of the conference was to bring astronomers who worked on planets in some forms over all of the HR diagram, be that studying the formation of planets at the very beginning of a stars life, observations of the debris discs around giant stars, or the remnants of planetary systems at white dwarfs. By trying to bring all of those disparate areas together, we can hopefully begin to fill in some of the gaps in the planetary HR diagram.

Over the next couple of posts I'll try and do a whistle stop tour of some of the talks at the conference, highlighting those areas I found interesting (/understood). And then I'm off to another conference... until then, new blogs will be posted on Twitter.  

Monday, 28 July 2014

How Dead Stars are Teaching Us What You Need to Build a Planet

What I actually do for living...

Repost alert! This first appeared as a guest post on Andrew Rushby's excellent II-I blog. I'm reposting it here as I'm off to a conference at Cambridge today, so will hopefully have quite a lot to write about soon. This post was written as an introduction to the research I'm doing for my PhD, so may be useful in providing some context for what I talk about next time. If you've already read it, feel free to stop here. If not, read on!

An asteroid plummets to its doom around the white dwarf GD 29-38. Studying the debris left from these asteroids can reveal the chemical composition of exoplanets. Image Credit: NASA

Twenty seven years ago astronomers noticed something strange about the white dwarf star GD29-38.

White dwarfs are dead stars, the burnt out carbon cores of stars like our Sun which have exhausted their hydrogen fuel; incredibly dense, incredibly hot balls of matter roughly the size of the Earth. Because of this high temperature, tens of thousands of degrees, all white dwarfs glow blue.

But the light from GD 29-38 wasn’t just blue. When it was split into a spectrum, separated into a rainbow of separate colours, there seemed to be something else there. Something shining with an infrared light, beyond the range of our eyesight.

Initially the discovers were excited, as the red light could have come from an orbiting brown dwarf, a mysterious object several times bigger than a planet but much smaller than a star. But both the white dwarf and the infrared source were pulsating slightly, periodically getting brighter and dimmer. If the red light was from a separate object, then it shouldn’t have pulsed in time with the white dwarf.

The spectrum also revealed metals in the white dwarf’s atmosphere, heavy elements like calcium, magnesium and iron. These were also out of place, as white dwarfs have such a strong gravity that anything heavier than hydrogen or helium should have sunk down into their cores long ago. The metals must be falling onto the white dwarf from the space around it- but how did they get there?

It took until 2003 for the origin of the mysterious infrared glow to be found, during which time many more white dwarfs with similar red spectra and metal polluted atmospheres were found. The explanation was that the infrared light is coming from a disc of dusty debris surrounding the white dwarf.

This debris was formed from the wreckage of an asteroid, leftover from when GD29-38 was a Sun-like star with its own system of planets. The dust in the disc rains down onto the white dwarf, explaining the metals we see in the atmosphere.    

The spectrum of GD 29-38. Along the bottom is its wavelength, or colour, going from blue on the left to invisible infrared on the right. The vertical axis shows how bright the white dwarf is at each wavelength. The difference between the blue white dwarf and red dust cloud can be clearly seen. Image Credit: NASA

The story of how the debris disc got there is a result of the turbulent formation of the white dwarf. As it runs out of fuel a star swells up to a huge red giant, then blows away roughly half of its mass in an immense stellar wind, leaving the tiny white dwarf core.

 With the gravitational force at its heart cut in two, the system of planets around the dying star is thrown into chaos. Planets begin to migrate outwards, trying to reach orbits twice as far away from the central star as before. As they do this, they risk coming into close contact with each other.

Some of the planets survive these encounters and carry on as they are. Others, especially when a big Jupiter sized planet is involved, are thrown out of the system into the depths of interstellar space. And some are scattered into the centre of the system towards the white dwarf. 

These unlucky asteroids and dwarf planets fall in towards the white dwarf until they reach a point known as the tidal disruption radius. There the tidal force, the difference in gravitational pull between the parts of the asteroid nearest the white dwarf and the areas further away, becomes so great that the asteroid is ripped apart, forming the dusty debris disc that we see as an infrared glow.

The discovery of this process lead to an important conclusion. As the dust rains down onto the white dwarf it becomes visible to our telescopes. If we can measure what metals there are, and how much of each there is, then we can reveal the chemical composition of the asteroid or planet that formed the disc. We can ask, and answer, the question: “What are planets made of?”

Two decades ago we only knew about the eight planets in our solar system (Pluto was never a planet, it was just mislabelled). Now we know of over a thousand planets, new worlds orbiting hundreds of stars. Through our telescopes we can measure the size of these planets, what their masses are, and even in some cases get a glimpse into their atmospheres.

But we can’t find out what they’re made of, what the geology of these newly discovered planets is like. This means that we don’t know for sure if the way that the rocky planets are built in our solar system, the particular mix of iron, oxygen, magnesium, silicon and other chemicals that make up the Earth and its neighbours, is the way all planets are built.

The metal polluted white dwarfs form a perfect laboratory, presenting us with rocky objects that have broken apart into their chemical components. By observing as many as we can, we can begin to explore the chemical diversity of planets and planetary systems. We can see if the way our planets are built is the normal way to construct a planet, or whether Earth is even more unique than we thought.

To date we’ve discovered around a dozen white dwarfs with enough chemicals to compare their systems in detail with our own. So far, they look fairly similar to the Earth, a hopeful sign. But we need many more to truly explore this area, and over the next few years myself and others will be scouring the sky, using the Hubble Space Telescope above us and an array of telescopes on the ground. We will find more metal polluted white dwarfs, measure the chemicals of the planetary debris around them, and begin to explore in detail what things you need to build a planet.