AAS #1: Zombie stars and planets kicked out of the crib

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Jan 5, 2009
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This week is the meeting of the American Astronomical Society, where lots of cool news is released. I am not attending, but I'll be reporting on some of the news released during the event. You can watch the press conferences live at the AstronomyCast live stream.

This morning we have two interesting results coming from the orbiting infrared observatory Spitzer Space Telescope.

1) Baby Jupiters kicked out of the crib early

Spitzer observations show that planets have to form quickly, before they run out of food.

Stars form from clouds of gas and dust, which collapse into disks (called protoplanetary disks). The star forms in the center of the disk, and planets form farther out. We have a gazillion examples of this; I worked on Hubble observing quite a few such systems. A big question in this field is, how long does it take for the planets to form? Eventually, the star's winds blow away the gas disk, and at that point the big gas giant planets are done; there's no more material for them to eat allowing them to grow. It's a bit like trying to get a building put together on a construction site before the foreman blows the whistle and tells you to go home.

Spitzer view of NGC 2362
Spitzer view of the young cluster
NGC 2362; click to embiggen.
Now we have an idea! Astronomers took a look at the young star cluster NGC 2362, known through previous observations to be about 5 million years old. What they found is pretty cool: stars with a mass of about the Sun's or higher don't have their big protoplanetary disks anymore, and only a few with less mass still have those disks.

Assuming the Sun and most stars form under similar conditions, this puts an upper limit on how quickly gas giant planets like Jupiter and Saturn planets can form: 5 million years. If they took longer, they'd never make it before the raw building materials were blown away.

I have to say, that's pretty fast! Objects like Jupiter are pretty beefy, and having it collect all that material in 5 million years (or less!) means it grows rapidly. The rate at which materials collect must heat those planets unimaginably hot!

Interestingly, the astronomers found that while the bulk of the gas disk gets blown away after 5 million years, there is still evidence of rocky material existing, which means that there is still some leftover bricks with which to build planets. These would probably help planets like Earth form, so it looks like gas giants form most of their bulk early, and while they can still grow some using that rocky material, that's not a big deal compared to the early growth. But Earth-like planets need not be so rushed; there is still plenty of material left over to form them.

So Jupiter may be more than Earth's big brother due to its size; it may actually have formed first, too! Just so you know, it has 300 times the Earth's mass, so it must have grown incredibly quickly. And now that it's so big, I'm glad it doesn't give us cosmic wedgies or anything like that. As big brothers go, Jupiter's pretty cool.

2) Astronomers using Spitzer Space Telescope have found that dead stars eat their kids.

Stars like the Sun eventually run out of fuel. When they do, they expand into red giants and shed their outer layers. After a few hundred million years all that's left is the exposed core of the star, compressed into an object called a white dwarf; a ball the size of the Earth with the mass of a star.

If the star had planets, it may have eaten the inner ones (like Mercury and Venus) during the red giant phase. But even long after the star is dead, it still feeds on the living: astronomers have detected white dwarfs consuming asteroids. Any of these rocky denizens that survived their star's death paroxysms may yet have a fiery fate. Gravitational interactions with other asteroids or any surviving planets can send the rock down to the star, where the ferocious gravity tears the asteroid apart, grinding it into dust. This dust can be detected in infrared spectra of the star.

Spitzer provides that sort of data, and astronomers have found eight such examples: white dwarfs that have clearly been feasting on asteroids. Two had been known previously, but this new result indicates that this event is common. It also shows that many stars have asteroid belts, itself an important result! Also, the asteroids orbiting these stars appear to be low in carbon, which is similar to the asteroids in our own solar system. That means that events leading to the formation of the Earth and other planets is likely common throughout space... something that indicates there may be more Earths out there.

So both Spitzer results show us that planets like Earth may be out there, forming commonly around Sun-like stars! Every day, we get a little bit closer to finding another blue-green planet like our own.

Image credit: Credit: NASA/JPL-Caltech/T. Currie (CfA)