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

By Phil Plait | January 5, 2009 10:50 am

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)

CATEGORIZED UNDER: Astronomy, Cool stuff, Science

Comments (18)

  1. Great summary Phil — as noted in the AstronomyCast Live chat room, I’ve been having Internet trouble. So mucho appreciated.

  2. David D

    OT I know, but I am surprised there are no posts about Jett Travolta’s death and how it may have been totally preventable if his parents weren’t Scientologists. Certainly has that anti-science vibe, don’t you think?

  3. Mchl

    There’s no confirmed information (or I did not find any) about whether Jett was or was not treated for his illness. Knowing ICS’s attitude towards medicine, we might assume he was not. But that is only assumption.
    And remember, people actually do slip in bathrooms.

  4. Todd W.

    Earth-like planets may be forming commonly? Aha! That makes it more likely that there is life out there, which, as anyone knows, means that aliens are among us! 😛


    This explains why our own Solar System has four rocky planets in the inner region, and four gas giants in the outer region — no, Pluto is not a planet!

    There… I’ve started that flame war, again!

  6. Todd W.

    So, what about ice planets? Any of those out there, or just icy moons?

  7. Well, we all know that Hoth exists, right? It was in that “Star Wars” series of documentaries. 😀

  8. Charles Boyer

    The discoveries in the past twenty years of astronomy are simply mind-shattering. One can only imagine what the next hundred might bring.

  9. Geochemical evidence from meteorites tells us that in our solar system, asteroid sized bodies form in less than a million years, but it takes 30-50 million years for those to collide to form planets.

    Wouldn’t the heat of tidally fragmenting an asteroid volatilize the organic component?

  10. Jacques Meade

    @ David D,

    What’s this post got to do with Jett Travolta’s death and his parents being Scientologists?!

  11. Sili

    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.

    More like before someone cuts the padlock and absconds with your bricks and trucks in the middle of the night (quite common here, I fear).

  12. David D

    Hey Jacques–

    I didn’t think this was (just) an astronomy blog. Did you?

  13. Daffy

    David D,

    No, but this thread is.

  14. David D

    Sorry–I even mentioned it was OT (that’s not “Old Testament,” by the way . . .

  15. @BA “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.”

    I would have thought that the distance from the star has something to do with the longevity of the gas/dust disk. We would expect the stellar wind to sweep the area within 1 AU clear of gas before it sweeps the entire area within 10 AU. It’s interesting that the article didn’t talk about distance as a factor and just focused on the number 5 million years.

    Some other data points are these:

    Beta Pictoris has an approximate age of 12 million years with a debris disk out to 1800 AU. Some interesting notes from Wikipedia:

    “Studies made with the NASA Far Ultraviolet Spectroscopic Explorer have discovered that the disk around Beta Pictoris contains an extreme overabundance of carbon-rich gas. This helps stabilise the disk against radiation pressure which would otherwise blow the material away into interstellar space. Currently, there are two suggested explanations for the origin of the carbon overabundance. Beta Pictoris might be in the process of forming exotic carbon-rich planets, in contrast to the terrestrial planets in our solar system, which are rich in oxygen instead of carbon”

    So apparently having carbon in the gas can stabilize it and make it last much longer than ordinary. I have no explanation for how that is supposed to work but it might explain why the disk hasn’t dissipated yet.

    Vega has an approximate age of 500 million years and a debris disk extending out to 800 AU. A perturbation has been found in the debris disk at about a distance of 70 AU which may be a planet during formation.

    So 5 million years is not a hard and fast rule for planetary formation. It depends on many different factors.

  16. dkary

    I didn’t see the press conference, but I did see a couple of other related talks on this subject.
    I haven’t been to AAS in a few years, and I’m really struck by the way debris disks and the transition from traditional protoplanetary disks to debris disks is now hot news.
    We’ve known for a long time that disks appear to disappear fairly quickly, with 1-10 million years a typical timescale. But now we’ve got clear enough measurements over enough wavebands to start testing specific mechanisms for clearing dust and/or gas.

    First off, Tom’s comment about Beta Pic, Vega, etc. These are all debris disks, and as such they have very little gas. So you could keep making terrestrials and icy planets, but gas accretion is not likely in any of these older systems. That’s one of the characteristics of debris disks.

    For the dust clearing, there are two competing approaches to removing the fine dust: photoevaporation, which does work from the inner disk outward, and efficient accretion which operates more evenly throughout the disk. Probably both were in operation, but at least one talk I saw today argued that the latter process dominated more than 50% of the time (at least in one star-forming region).

    On a related note, the whole discussion of extrasolar planets has just exploded in the last several years. Once upon a time this was a couple of talks at the end of the planetary sessions (itself a small part of the AAS meeting). Now there are several sessions devoted just to planets around other stars in both the poster and oral talks. It almost makes me wish I’d stayed in research.

  17. @dkary “First off, Tom’s comment about Beta Pic, Vega, etc. These are all debris disks, and as such they have very little gas. So you could keep making terrestrials and icy planets, but gas accretion is not likely in any of these older systems.”

    Well, the article the Phil linked to says the following:

    “Kenyon added that while Earth took about 20 to 30 million years to reach its final mass, Jupiter was fully grown in only 2 to 3 million years.”

    That sort of gives the impression that the dust debris disk is only around for maybe 30 million years or so (enough time for Earth and the other terrestrial planets to form) and then it’s pretty much gone. That’s why I gave the example of Vega which is half a billion years old and still has a sizeable debris disk with possible planet formation going on right now. So I am a bit skeptical of some of these low time ranges being handed out. Perhaps the computer models say it works that way but in reality we find much older debris disks.

  18. dkary

    There seems to be quite a range of lifetimes for debris disks. Most stars seem to lose them in less than 100 million years, but there are a few (like Vega) that have them for much longer.

    I wasn’t too happy when the first observed estimates of disk lifetime came out back in the 90’s and they were running around 10^6 years. It now looks like times of a few million years are fairly common for the gas, and that solid debris can last a lot longer than that. These are quite consistent with models of both giant and terrestrial planet formation.


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