How short can a planet's year be?

[Robh]

The planet 55 Cancri e orbits its sun in just over 17 hours.
http://www.newscientist.com/article/dn18979-how-short-can-a-planets-year-be.html?DCMP=OTC-rss&nsref=online-news

More here ...
http://en.wikipedia.org/wiki/55_Cancri_e

By my calculations, assuming its sun has the same mass as ours, its orbital radius would be about 2.4 million kms. In comparison Mercury, orbits our Sun at about 58 million kms.
I'm surprised that a planet could form this close to its sun.

Rob.

[renormalised]

It didn't form close to its sun. The planet would've migrated in as it was forming, or after it fully formed and was interacting with the last remnants of the accretion disk and the other planets...if it what's left of a gas giant. If it's a terrestrial planet, then it probably did form where it is, or close by, or it maybe an ice giant, like Neptune, that's undergone the migration route.

55Cancri, btw, is smaller than the Sun...it's G8.

[renormalised]

There are many factors which determine where and whether a planet will form in the accretion disk around a young star. It's a matter of composition of the disk, the density of the materials, the dynamics of the particles within the disks, their size, the temperatures within the disk and how the materials separate out due to the temps, etc etc etc.

If it did form close in, the 55Cancri e could be nothing more than a huge chunk of metal surrounded by a dense, but thinnish, atmosphere of hydrogen and helium and whatever else. When I mean thinnish, probably only 1000km or less in thickness. In so close, the only materials which could survive would be things such as iron, nickel etc. Silicates would have a hard time forming...the planet would have to be at least 1/2 the distance to Mercury out from its star to have the chance to form stable silicates.

[mithrandir]

55 Can A is the larger member of a binary. In mass maybe smaller then Sol, but the range for the margin of error includes 1 solar mass. The radius is alleged to be greater. At least that's what this (http://en.wikipedia.org/wiki/55_Cancri) wikipedia link says.

The other half, 55 Can B, is M3.5-4.

[renormalised]

I've read it...big problem with it....the mass and the radius don't jibe with the stellar theory...1.03M and 1.15R for a G8 star...hmmmm....that's more like a star of G1. However the anomalous spectral type is most likely due to the high metals content of the star.

The star is about 0.89L which is 167% brighter than it should be (should be 0.53L for a normal G8 star of its age and temp etc), given the latest references. If that star didn't have such a high metal content, its surface temp would be around 5900-5950K and the star would be just over 1.4L, which is pretty much bang on for a G1 star.

[Robh]

Thanks for the info.
If its a "small" gas giant like Neptune and migrated inwards, is it going to eventually be absorbed by 55 Cancri A or will solar winds carry its gases off into space.

Rob

[renormalised]

It should be pretty safe considering it has a circular orbit. If its orbit was still markedly elliptical (say e>0.02) it would likely fall into the star, or if it preceded the tidal bulge it raised on the surface of the star by a long way, it might lose orbital energy. Most of its problems would be in losing atmosphere to ablation by heating, being so close to the star. Ablation by the stellar wind may happen if it doesn't have a powerful magnetic field...at least one powerful enough to protect the atmosphere. It's hard to say what will eventually happen until they do more work on the system. Usually, gas giants are pretty safe even in torch orbits because their gravity is sufficient enough to hold onto their atmosphere, and they have powerful magnetic fields.

[Insane Climber]

Arhm scuse me but if its orbit is so fast wouldn't the inward spiral be non detectable buy us.

[renormalised]

On the contrary, it would be even more noticeable. The faster it moved in its orbit, the closer it'd be to its parent star, the quicker tidal forces would do their job and it'd begin to fall into the star. However, before it even fell into the star's photosphere, the planet would get ripped apart and partially vapourised. What was left of it would form a (very) temporary disk around the star.

[Insane Climber]

That's very interesting. But i was thinking there must be a ratio, that we could calculate. As to the minimum distance, that a certain sun/planet masses could be. before this would happen.
Is it OK to just use the old centrifugal force equation and calculate how fast it would have to go before the gravitational "string" would balance? then work back until you found where the point is that the planet would either pull apart or fall into the sun.??

[sjastro]

It is based on the Roche limit where the gravitational force on the body is equal to the tidal force on the body. The Roche limit involves a number of factors such as the rigidity of the body, the mass of the Sun and the body, and the radius of the body.

The tidal force is simply the difference in gravitational force exerted on a body. For example the sunlit side of Mercury is subjected to a higher gravitational force than the opposite side which is further away from the Sun (inverse square law).

It is possible for the Roche limit to be inside the Sun for a given body in which case a body will never break up irrespective of how close it is to the Sun.

Regards

Steven

[Robh]

Thanks for that, very interesting. According to theory, for a rigid spherical body that is more than twice the density of the primary, the Roche limit will be inside the primary. I notice the Roche limit for a solid Earth is 0.8 x Sun's radius but for a fluid Earth it is 1.53 x Sun's radius. (Wikipedia on Roche Limit). There must be some point outside the Sun where a solid body would become fluid due to heat radiation. I wonder how far out this point would be?

Regards, Rob.

[Insane Climber]

Thanks Steven, the idea of the Roche limit has filled in a bit of knowledge for me :) I do suffer from thinking of it like Ballard Balls.

[Astro78]

There might be people made from silica tetrahedral bonds, kinda like semi-morphing crystal people ..