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.
