orbits

[wolram]

http://www.ucolick.org/general/pressrelease/99/99-11-29.html

The orbits of the new planets, like those of most of the extrasolar planets discovered so far, tend to be quite eccentric, tracing paths that are oval rather than circular. One of the planets, around a star called HD 222582, has the most wildly eccentric orbit yet known, carrying it from as close as 0.39 astronomical units (AU: the distance from Earth to the Sun) to as far as 2.31 AU from its parent star in the course of its 576-day orbit. “It is beginning to look like neatly stacked, circular orbits such as we see in our own solar system are relatively rare,” Vogt said.
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maybe this is bad news for seti, but how eccentric could
a planets orbit be and still support inteligent life?

[Raven]

One of the planets around the star called HD 222582 (the wildly eccentric one) seems to have an orbit more like that of a short course comet than that of a planet.

[Nereid]

wolfram wrote: maybe this is bad news for seti, but how eccentric could a planet's orbit be and still support inteligent life? Perhaps we could consider this from two perspectives:
- how does intelligence arise?
- what about life?

Taking the second first. If most life on Earth is in the crust and not on the surface, it would seem that carbon-based life could exist on a planet (or satellite) with any orbit which didn't cause the crust to get too hot, say <200oC. The constraints are more on the geophysics and geochemistry of the planet.

The first question is much more difficult, since we really have no idea what conditions give rise to homo sap., the only intelligent life we know of. It could be that periodic bombardment by asteroids is critical, for example. Limiting the question just to eccentricity of orbits, and assuming critters have to be big to be intelligent, some weak boundary conditions might be:
- not so eccentric that the planet freezes over entirely (no big critters live in Antartica)
- that ground surface temperatures remain above 50C for months at a time (we really don't know; big critters don't live in deserts more 'cause it's dry than
'cause it's hot)
- the oceans don't boil

[Adrian Baker]

Near circular orbits are more likely to form from a collapsing cloud of stella debris than very elliptical ones. The Solar mass at the centre will determine the orbital periods of the cloud particles orbiting it. Local gravitiational effects will then take over as larger and larger masses form. As these orbit, they will 'hoover' up the particles left over, forming planets in roughly circular orbits. (NB This is a VERY simplistic explanation!)

For the kind of exo-planets we are discovering elsewhere, only the very large ones are detectable. These tend to have formed as 'failed' binary star systems. ie the large 'non-star' is what we discover as a planet, orbiting the sister star. These two will orbit around a common centre of mass, looking to us like a planet with a very elliptical orbit, orbiting a star.

[russ_watters]

First, that page is pretty out of date at 4 years old. Second, 28 is not a very large sample to reach such a conclusion from. Third, detection methods aren't good enough yet to detect anything but large planets orbiting (or just passing) close to their stars.

[Nereid]

In my first post I didn't consider waterworlds, planets which are composed largely of 'volatiles' rather than rocks. The Galilean satellites of Jupiter (other than Io) are good examples of such bodies.

Such a planet would be able to retain a sizable ocean under a wide range of possible orbits, including many eccentric ones. Seems to me it would just need to be big enough to retain a decent atmosphere when near its parent star, and have had sufficient radioactives in the rocky core (U, Th, etc) to keep the deep ocean liquid over a few billion years.

Whether intelligent life could arise on such a planet is an entirely different question!

FYI, this is a good, up-to-date compilation of all findings (including some questionable ones), along with a summary of the methods currently used to find extrasolar planets:
http://www.obspm.fr/encycl/encycl.html