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Fraser
2006-Oct-12, 12:14 AM
Many extrasolar planets have been discovered circling other stars, a few of which are 5-15 times the mass of the Earth, and thought to be solid like our planet. Astronomers were surprised to find these planets orbiting small, cooler red dwarf stars. Researchers believe these "super Earths" form in the chilly halo of snow, ice and frozen gasses that collect around red stars as they cool. There probably isn't enough solid material to form rocky planets much larger than Mercury in the star's habitable zone.

Read the full blog entry (http://www.universetoday.com/2006/10/11/super-earths-emerge-from-snowy-conditions/)

grantkennedy
2006-Oct-12, 03:15 AM
After reading Universe Today for such a long time it's an honour to be on it. Thank you!

WizardWayne
2006-Oct-12, 03:33 PM
It seems to me that since as many as half of the stars are double (or multiple) that the same should hold for dwarf stars, both the red and brown variety. Large planets around red dwarfs may not be a "planet" in the sense that it fromed by accretion from the leftovers of the formation of the star but rather by direct gravitational collapse as the star itself did, in effect, a very small binary companion.

antoniseb
2006-Oct-12, 04:34 PM
Large planets around red dwarfs may not be a "planet" in the sense that it fromed by accretion from the leftovers of the formation of the star but rather by direct gravitational collapse as the star itself did, in effect, a very small binary companion.

You might say the same thing for Jupiter and Saturn in our system. It seems unlikely for a rocky planet though.

Leafguy
2006-Oct-12, 05:48 PM
I don't think you could say the same thing for Jupiter and Saturn. I suppose it is possible, but looking at the rest of the bodies in the solar system, I don't think any of the terrestrial planets could have formed in that type of environment with a developing sun, and a potential binary star system early on in the solar system formation. There would be far to much hostile activity to form any sort of sustainable planetary disk that would last long enough to form the inner planets without being sucked up.

BillG
2006-Oct-13, 03:25 PM
Antoniseb, why do you think that Jupiter and Saturn are more like binary companions of the Sun than like planets that formed by accretion of planetessimals? I think you'll find that current conventional wisdom is that both Jupiter and Saturn originally formed as rocky planets similar to Earth, but because they're out past the frost boundary they were able to accrete huge amounts of ice crystals onto themselves and thus grew very large by a process called nebular accretion.

antoniseb
2006-Oct-13, 03:48 PM
Antoniseb, why do you think that Jupiter and Saturn are more like binary companions of the Sun

Actually, my thought is a little more hybrid than just a simple binary companion idea. Looking at Jupiter (ignoring Saturn for the moment), the isotope ratios and atomic abundances for Jupiter seem to be quite similar to that of the interstellar medium and the Sun's photosphere, whereas the inner planets seem to have a more evolved isotope ratio. Also, Jupiter has so much more mass than the other planets that it seems a little out of place, and some explanation would be required as to why the vast majority of the mass of the protoplanetary disk ended up in Jupiter.

On the other hand, there is no reason that there should not have been multiple pre-stellar cores in the process of condensing in the vicinity of the Sun when condensation got turned off by a nearby supernova blast, or perhaps radiation from a B or O giant. What would happen to a Jupiter mass core after that? If it formed within a small number <30 of AU from the Sun, I think we'd eventually see something like Jupiter.

Do I have proof? No. I'm just looking at this as a reasonable explanation. When there are observations that say it can't be this way, I'll change my story.

John Mendenhall
2006-Oct-13, 05:03 PM
I'm with Antoniseb. In the solar system, Jupiter dominates. I once saw the solar system described as "The Sun, Jupiter, and assorted debris." I wish I could credit the quote, but it's been at least 30 years since I read the remark.

BillG
2006-Oct-14, 05:06 AM
Jupiter has so much more mass than the other planets that it seems a little out of place, and some explanation would be required as to why the vast majority of the mass of the protoplanetary disk ended up in Jupiter.


I think the generally accepted explanation of that in the Planetary Science community is that Jupiter is the first planet outside the frost boundary, where all the volatiles that had been in the inner solar system were pushed by the protosun's T-Tauri wind. They got out there, froze into solid crystals of ice, and were available for accretion onto whatever planet happened to form.

I'm not a Planetary Astronomer myself, so I won't claim expertise. But I do teach this stuff and that's what you'll find in the current textbooks.

grantkennedy
2006-Oct-16, 05:02 AM
A couple of things...

I think it would be hard to justify one of these super-Earths created by our model (and observed by microlensing (http://www.nature.com/nature/journal/v439/n7075/index.html)) as coming from gravitational instability in a disk, since this method tends to make much larger gas giants. There is the idea that a gas giant formed this way could later have it's atmosphere removed by a nearby massive star (here (http://arxiv.org/abs/astro-ph/0605061)) but I think this probably makes planets closer to Neptune size, five Earth masses is probably near the bottom of the range and harder to justify. On the other hand, five Earth masses is at the upper end of what our model can form...

Regarding Saturn's size, there are a few reasons why it might be smaller than Jupiter, without Jupiter needing a disk instability mechanism. As noted above the amount of dirt and ices may have been greatly enhanced just outside the snow line. Another reason may be that Saturn started attracting gas later than Jupiter (further out = slower core formation), and thus had less time before the gas disk was removed. Finally, the distance at which most gas was removed from the solar nebula may have been near Saturn, thus hindering its growth.