Smallest extrasolar planet found

[J'Dona]

Astronomers have detected the smallest extrasolar planet yet: a world about seven and a half times as massive as Earth orbiting a star much like ours. The new find may be the first rocky world found around a star like the Sun. It orbits the star Gliese 876, located 15 light-years away in the direction of the constellation Aquarius, and orbits with a period of 1.94 days at a distance of 0.021 AU, or 3.2 million km.

 

http://news.bbc.co.uk/1/hi/sci/tech/4089534.stm

 

Because its mass is so small, it is not thought to be able to retain gases like hydrogren in the way that a gas giant does.

[AzurePhoenix]

The fact that it's very, very close to its star also doesn't really help its potential for having much of an atmosphere either.

[Ophiolite]

This mass is in the range we would expect for the rocky core of a Jovian planet. Given the known predeliction for gas giants to migrate inwards, I suspect this is one that has done so, then lost its atmosphere through the joint effects of high temperature and stellar wind.

[Spyman]

They, (the discoverer's), seems to think it could have an atmosphere ...

 

According to astronomer Geoff Marcy, "... We have no analog like this in our solar system. We do not know its composition - whether it is all rock or some chimera of rock, ice, and a thick atmosphere - perhaps a hybrid of Earth and Uranus."

http://www.spaceref.com/news/viewnews.html?id=1031

 

"The planet's mass could easily hold onto an atmosphere," noted Laughlin, an assistant professor of astronomy at UC Santa Cruz. "It would still be considered a rocky planet, probably with an iron core and a silicon mantle. It could even have a dense steamy water layer. I think what we are seeing here is something that's intermediate between a true terrestrial planet like the Earth and a hot version of the ice giants Uranus and Neptune."

http://www.universetoday.com/am/publish/large_rocky_planet.html?1362005

 

Whether it has an atmosphere at all is unknown. It could consist of bare rock, rock swathed in an atmosphere as thick as Earth's or Venus's, or it could be a Uranus analog, a hot "ice giant" in which a massive atmosphere rich in cosmic volatiles (water, methane, ammonia) comprises a fair fraction of the objects's entire bulk.

http://skyandtelescope.com/news/article_1530_1.asp

[Ophiolite]

Good information Spyman. I allowed myself to be misled by J'Dona's comment "a star like the sun". While it is true Gliese 876 is main sequence star, it is an M4, as opposed to the sun, which is a G2. So Gliese 876 is 30% of the mass of the sun, has a lower surface temperature (3300K, versus 5200K), but most significantly has a luminosity less than 1% or 2% of the sun. So, although the planet is in such a close orbit it is indeed entirely possible that it could have retained an atmosphere.

[alibabba]

This mass is in the range we would expect for the rocky core of a Jovian planet. Given the known predeliction for gas giants to migrate inwards, I suspect this is one that has done so, then lost its atmosphere through the joint effects of high temperature and stellar wind.

 

Why do gas giant planets migrate inwards and rockey planets do not?

[calbiterol]

Why do gas giant planets migrate inwards and rockey planets do not?

 

Or, alternatively, do rocky planets tend to migrate towards their sun(s) as well?

[Ophiolite]

Good questions. (I always say that when I am not sure of the answer, since it often distracts the questioner long enough for me to slip past in my cloak of ignorance.) I believe there are two factors at work for Jovian planets:

a) Gravitational interactions

b) Frictional effects

 

We are accustomed to think of solar systems as nice orderly arrangements of planets, orbiting for billions of years like clockwork. This was decidedly notthe case when the systems form from a protoplanetary disc. This is a time of violence and upheaval on a scale we can describe, but cannot truly imagine.

In this setting interactions between giant planets may cast some of them out of the system entirely, while the others, having lost angular momentum in the exchange, must necessarily move inwards. This has been extensively modelled, and matches quite well the observed positions of many of the extra-solar planets observed to date.

Also, at the time of planet formation, the plane of rotation of the system is crowded with gas, dust, particles, meteors, asteroids, comets, planetismals. The collisions between these tend to convert momentum into heat, with a result that the planets are slowed by a tiny amount, which over time can become a large amount.

Now, I'll go away and research the correct answer.

[On the topic of rocky planets, I don't think we know for sure. I think the theoretical answer is yes, but perhaps not as much as the giants, but until this one, we haven't really observed any extra-solar rocky planets in order to form an opinion.]

 

Edit: "The accreted matter has less orbital angular momentum than the planet and exerts an effective inward torque, so that inward migration is slightly accelerated. " from http://www.phys.lsu.edu/~andy/reprints/planet2.html

[J'Dona]

Good information Spyman. I allowed myself to be misled by J'Dona's comment "a star like the sun". While it is true Gliese 876 is main sequence star, it is an M4, as opposed to the sun, which is a G2. So Gliese 876 is 30% of the mass of the sun, has a lower surface temperature (3300K, versus 5200K), but most significantly has a luminosity less than 1% or 2% of the sun. So, although the planet is in such a close orbit it is indeed entirely possible that it could have retained an atmosphere.

My apologies for the confusion; that particular phrase was from the BBC article. I really should have searched for a scientific article on the new planet like Spyman's before posting, rather than the first article I found.

[Ophiolite]

No apologies necessary. It motivated me to do a search that led to downloading 500 pages worth of fascinating papers on giant planet migration which I was still trying to digest at 3.00am this morning. I think this area of astronomical research is intrinsically more interesting than cosmology (where it seems you can postulate just about anything since it is so difficult to challenge) because we are on the verge of major breakthroughs in understanding of planetary formation mechanisms and the implications they have for the development of life. When Kepler gets working in 2012(?) it will be tremendously exciting.