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« How Do You Enforce It? | Main | On Track? »

Extrasolar Planet Next Steps has a good summary of the recently discovered extra solar planet massing only five times as much as ours.

My recommendation for the planet finders is to start looking for wobbles on the wobbles of the super massive planet orbits to see if they can find smaller planets or Moons. Or wobbles on cold binary stars that circle near the hab zone of hotter primaries that may also turn up lower mass planets.

Even if we never directly detect low mass planets, big hab zone planets may be like Jupiter or Saturn and have lots of moons, some of which have comfortable gravity and an atmosphere.

Posted by Sam Dinkin at February 02, 2006 01:05 AM
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Considering that many of these probable moons around large exoplanets may be like Europa, they could be very good candidates for life.

All we could discern from this distance would be a confirmation of spectroscopic water signatures (if we had such sensitive instruments) and the cold temperature based on it's distance from the primary star.

We may have a flourishing life-bearing galaxy, but most of it's under a thick shell of ice.

Posted by B.Brewer at February 2, 2006 06:43 AM

If a moon is in a close orbit around a large exoplanet, that moon will have an enhanced rate of large impacts. The planet will gravitationally focus impactors, and if the moon is close to the planet the impact velocity will be increased as the impactors fall into the moon's gravitational well (and the moon swings around the planet at high speed).

This same effect will cause lone planets in habitable zones around cooler stars (K, M dwarfs) to experience more energetic impacts from impactors of a given size.

Posted by Paul Dietz at February 2, 2006 07:05 AM

Not to mention being roasted by radation trapped within the massive magnetic field of the Gas Giant unless they ae in a very high orbit.

Posted by Mike Puckett at February 2, 2006 07:24 AM


I recall reading that Tau Ceti may be filed with giant asteroid fields rather than planets. A millenia from now (maybe less) experienced settlers who cut their teeth on our asteroid belt may be ideal candidates to begin extra-solar colonization.

Eyes on the prize, and all that.

Posted by Bill White at February 2, 2006 08:34 AM

If only the greys would just tell us where their planet is, and we could use our observations of it to look for others...

Oh yeah, Zeta Reticuli, right?

Posted by Astrosmith at February 2, 2006 12:00 PM

I've always wondered why gas giants themselves are not considered as homes for life, except in science fiction. They have a thermal and pressure gradients ranging from cold vacuum to superhot/superdense, water, and organic molecules. Somewhere in there, the pressure is one atmosphere; somewhere else perhaps, the temperature is 20 degrees C. I guess if these altitudes are far apart, that might be a problem for me, but there must be areas in Jupiter, for instance, where some type of life could evolve.

Any enlightenment most appreciated.

Posted by Patrick at February 2, 2006 12:36 PM

It may mostly stem from a difficulty in visualizing life starting where no useful solid surfaces and/or masses of liquid water exist. Without them, you may get lots of 'organic' material as you describe, in atmospheric suspension (which may well include copious water vapor and droplets), but perhaps nothing in which the pathway to cells or more complex life could happen.

But the Universe has so far proven to be full of suprises and I've been wrong before...

Posted by Frank Glover at February 2, 2006 02:00 PM

FWIW, I dug out my college physics textbook and did some back-of-the-envelope calculations about the gravity on that extrasolar planet, given that its mass is five times that of Earth (around 3*10^25 kilograms) and assuming the planet is more-or-less spherical (a good bet).

If it is "an icy world", as the article suggests, then the overall density of the planet should be around that of water ice (approx. 1000 kilograms per cubic meter). This gives a volume of 3*10^22 cubic meters, and a radius of 1.9*10^7 meters. The acceleration due to gravity on the surface of the planet works out to 5.3 meters per second per second, or 0.55 times that of Earth.

If, on the other hand, the planet is rocky, we could assume that the density is close to that of Earth (although we may have to hold our noses while we make that assumption). In that case, the volume of the planet would also be five times that of Earth, and the radius would be the cube root of five (approx. 1.7) times that of earth, or around 1.1*10^7 meters. The acceleration due to gravity in this case works out to 17 meters per second per second, or 1.7 times that of earth.

You may note that, in both cases, the planet may be sutiable for exploration by human astronauts, although the 1.7g case would take some getting used to. Who knows, humans may even get there sometime in the next few millenia.

(And yes, I *do* have no life...thanks for noticing.)

Posted by Peter the Not-so-Great at February 2, 2006 11:29 PM

Peter: you cannot assume the density of ice inside this body will be the same as the density of ice at zero pressure. Planetary sized bodies experience significant internal compression. See the water/ice phase diagram here. The density of high pressure polymorphs of ice can be more than 2.5 g/cc, although this varies depending on the temperature.

Posted by Paul Dietz at February 3, 2006 05:13 AM


Good point. Peter's BOTE calcs do provide rough bounds, though, and point out that, although the planet sounds massive enough to have crushing gravity, worlds like it may be OK for humans. (We'll leave out all the issues of how much atmosphere such a world might hold on to at a more temperate distance from it's star.)

- Eric.

Posted by Eric S. at February 3, 2006 09:17 AM


I've been thinking about this exact topic. Folks fuss about tidal locking of Earth-like worlds around red dwarfs. However, an Earth-like moon of a Jovian planet in the red dwarf's hab zone would still have a 'day' of some dozens of hours. There's the whole flare thing to still worry about, but the Jovian's magnetosphere (and the moon's own magnetosphere as well) might just fend off the worst of the particles. Sounds like a good modeling project for some grad student...

- Eric.

Posted by Eric S. at February 3, 2006 09:22 AM

If you're looking for habitable moons of these things, "looking for wobbles on the wobbles of the super massive planet orbits" isn't going to do you any good. Remember that you don't see the planets directly. All you are seeing is the reflex motion of the star. So the masses of any moons have already been included in the inferred mass of the planet.

Now there is an off chance that you might detect a moon by the transit method, but, given that the planet only drops the observed flux by a few parts in a thousand, a moon is probably undectable. Besides, the transiting planets are so close to their stars that one wouldn't exactly call them "habitable".

Posted by The Observer at February 3, 2006 09:28 AM

Another point to add: retaining an atmosphere is a function of escape velocity, not surface acceleration, so a large, low density body could retain a breathable atmosphere even if its surface gravity were rather low. On the flip side, getting into orbit from the surface of a low density, 1 gee planet (such as, for example, Saturn) would be more difficult than from Earth.

Posted by Paul Dietz at February 3, 2006 09:51 AM

Paul: You're right, my assumption of the planet's density in the "icy world" case was too simplistic. Forms of ice with signifcantly higher density start showing up even at relatively low pressures (e.g. the ice-VI allotrope has 30% higher density, and shows up at around 6000 times atmospheric pressure). The net effect would be a smaller radius and a higher surface acceleration due to gravity.

As for escape velocities, they work out to about 14.5 kilometers per second for my (crappy) icy world case, and 19 kilometers per second for my rocky world case; by comparison, Earth's escape velocity is 11 kilometers per second.

Posted by Peter the Not-so-Great at February 3, 2006 11:50 PM

The speeds of the orbits will speed up and slow down based on moons and other small planets which will shed some light on what sorts of 2nd order grav anomolies are there.

Posted by Sam Dinkin at February 4, 2006 05:44 PM

Has anyone seen any type of plans published for what happens AFTER we find Earth-similiar planets with the Kepler and TPF missions?

For that matter, would any of you experts want to outline what type of missions you'd like to see happen after we find "Earth 2" (pardon the pun)?

Manned or unmanned? Generation ship or cryogenics? I recently saw that DVD that projected a Earth-like planet ~6 LY away and had hydrogen-filled AI-capable mini-blimps flitting around the surface. If memory serves, even with nuclear engines, it took somewhere on the order of 40 yrs at .2 c to get there.

I realize we need a drastic overhaul in propulsion to get to any planet closer than 25LY - it would be nice to see results in my lifetime.

Posted by Bob at February 6, 2006 06:26 AM

Has anyone seen any type of plans published for what happens AFTER we find Earth-similiar planets with the Kepler and TPF missions?

Visiting such planets would be so far in the future that planning for it would be an complete waste of time. This is pretty much pure science right now.

Posted by Paul Dietz at February 6, 2006 09:31 AM

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