Exotic Earths

More than one-third of the giant planet systems recently detected outside our
solar system may harbor Earth-like planets, according to a new study by
scientists associated with NASA's Astrobiology Institute. Many of these planets
may be covered in deep global oceans, with abundant potential for life.

PS: Seed Magazine news story on this

see WaPo's Achenblog for perspective:
Question Authority! Why?

Good BBC article on it here

Actual paper is up on Science's web site now (subscription)

Also Cool paper by Squyres' et al on water on Mars


Click here for hi-res version
© Nahks

The study focuses on planetary systems that contain "Hot Jupiters": gas giant
planets that orbit extremely close to their parent stars -- even closer than
Mercury to our own Sun. Hot Jupiters are believed to have migrated inward
toward their parent stars just as the planetary systems were forming, disrupting
the space environment and triggering the formation of ocean-covered, Earth-like
planets in a "habitable zone" conducive to the evolution of life, according to
the new study. Titled "Exotic Earths: Forming Habitable Worlds with Giant
Planet Migration," the Science paper was authored by Dr. Sean Raymond of the
University of Colorado at Boulder, Avi Mandell of Pennsylvania State University
and NASA's Goddard Space Flight Center in Greenbelt, Md., and Dr. Steinn
Sigurdsson of Pennsylvania State University.

Published in the Sept. 8 issue of Science, the study indicates Hot Jupiters push
and pull proto-planetary disk material during their journeys, flinging rocky
debris outward where it is likely to coalesce into Earth-like planets. At the
same time, turbulent forces from the surrounding dense gas slow the small, icy
bodies in the outer reaches of the disk, causing them to spiral inward and
deliver water to the fledgling planets. These planets may eventually host
oceans several miles deep, according to the study.

"These gas giants completely shake up the system as they migrate, but eventually
things settle down," said Mandell. "We now think there is a new class of ocean-
covered and possibly habitable planets in solar systems very different from our


Scientists had previously assumed that as Hot Jupiters plowed through proto-
planetary material on their inward migrations toward their parent stars, all the
surrounding material would be vacuumed up or ejected from the system. "The new
models indicate these early ideas were probably wrong," said Raymond.

The research team ran exhaustive simulations lasting more than eight months each
on more than a dozen desktop computers, starting from a disk of more than a
thousand rocky and icy protoplanets about the size of the moon. The initial
conditions for each computer model were based on current theories of how planets
formed in our own solar system and simulated about 200 million years of
planetary evolution.

In addition to the Earth-like planets that form in habitable zones outside Hot
Jupiters, the simulations showed some rocky planets known as "Hot Earths" often
form with orbits interior to the orbit of a Hot Jupiter. Evidence of this
phenomenon comes from recent discoveries of similar planets in other star
systems: planets more massive than Earth but much smaller than gas planets
(hence the term "Hot Earth"). These planets would be extremely hot and most
likely uninhabitable.

The new simulations show both Hot Earths and Earth- like planets in habitable
zones form with large amounts of water -- up to 100 times the water present on
Earth today. The models indicate such water-rich planets would likely contain a
lower percentage of iron than Earth, thought by some astrobiologists to be
important for the evolution and possible oxygenation of evolving atmospheres.
According to the team's computer simulations, Hot Earths can form astoundingly
fast -- in just 100,000 years or so, while the Earth-like planets in habitable
zones formed much more slowly, taking up to 100 million years. Geologists
believe Earth took about 30 million to 50 million years to fully form.

"I think there are definitely habitable planets out there," said Raymond. "But
any life on these planets could be very different from ours. There are lot
evolutionary steps in between the formation of such planets in other systems and
the presence of life forms looking back at us."

The team concluded that approximately 1 out of 3 of the known planetary systems
could have formed as-yet-undetected Earth-like planets in so-called habitable
zones similar to the one that Earth's orbit resides in. "The fraction of known
systems that could have the potential for life may be significantly higher than
we had thought," said Mandell. A whopping 40 percent of the roughly 200 known
extra-solar planets are Hot Jupiters, but this percentage will likely decrease
as more extrasolar planets are detected.

The new collaborative research effort may allow planet hunters to determine
rough limits for where to search for habitable planets in known systems of giant
planets. "We hope other researchers may be able to use our new model to narrow
the list of potential targets in the search for other Earths", Dr. Sigurdsson
said. This will help in upcoming space missions such as NASA's Kepler and
Terrestrial Planet finder and ESA's COROT and Darwin, which hope to discover and
eventually characterize Earth-like planets around other stars, the team writes
in Science. "We predict that a significant fraction of systems with close-in
giant planets will be found to have a Hot Earth or potentially habitable, water-
rich planets on stable orbits in the Habitable Zone. Suitable targets may be
found in the known giant planet systems."

Surf's Up, Dude


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Interesting study. My only quibble is what do they mean by habitable? Not in the sense that humans could live there, I'm guessing. From my admittedly limited understanding of the subject, waterworlds wouldn't be very cozy homes for complex life. There would be little circulation of nutrients in the global ocean, and without dry land, little way to regulate greenhouse gases. So all we are talking about here is something like alage on the surface with maybe some more complex organisms in thermal vents below, right? Has there been any serious look at what kind of life an ocean world could harbor?

Grats! Looking forward to your talk tomorrow :)

Walter, aren't you being way too pessimistic there? What would prevent fish from evolving in a waterworld? Or a Kevin Costner from living there? ;)

By JustKidding (not verified) on 07 Sep 2006 #permalink

Walter -

I think this is a problem; the shallow marine/coastal environments are very important for evolution on earth, and a waterworld may not have such things.

I say 'may not' - it would depend on a number of things. A hotspot volcano underwater can build an extremely high mountain, as could plate tectonics, so I'd expect some land even with perhaps 5-10x as much water as earth.

The GHG argument is quite interesting. A waterworld with no way of CO2-weathering drawdown would become extremely hot, which could lead to the oceans boiling, or at least transporting huge amounts of water vapour into the amosphere, at which point UV-breakdown would lead to hydrogen loss into space. So it could be plausable that a waterworld would be unstable, and would progressively lose water until rocks were exposed, and CO2 drawn down. Anyone put that in their model yet?

By Andrew Dodds (not verified) on 08 Sep 2006 #permalink

how much solid matter is there in a Jupiter? could Mercury be the boiled-down remnant of a gas giant that migrated in?

jb--You wouldn't easily boil down a Jupiter at Mercury's distance, as Hot Jupiters get much closer to their suns and seem to survive to ~Solar System lifetimes. There can be mass loss, and some planets may be whittled away to very small masses. Still an open topic of research.

I thought the solid core of Jupiter was more in the Earth mass range.

Of course, if you have too much water then you end up with an ice layer between the ocean and the silicate core, which could mean the ocean is extremely poor in minerals. I'd expect the majority of ocean worlds to be sterile.

Of course, if you have too much water then you end up with an ice layer between the ocean and the silicate core, which could mean the ocean is extremely poor in minerals.

What would the geology of the ocean floor be for such a world? I see no reason the core couldn't still be hot. For a given amount of water, the ice layer is more likely to be thicker when there is a more massive core. The gravity at a given radius will be higher if there is a massive core sitting inside that radius than if it's mostly water all the way down. To support hydrodynamic equilibrium, the pressure at that depth will have to be higher, pushing it closer to one of the ice phases. As I understand it, a larger core would be a hotter core, all other things being equal.

So, I'd expect there'd still be a lot of volcanism at the ice-core interface. Would any of that translate to activity at the water-ice interface? If the ice covering a volcanic vent is heated enough, then according to the phase diagram, it should melt (at pressures below 10 GPa ~ 10^5 atmospheres). Could a volcano at the core surface create plumes of water through the ice? If not, might there be pockets of extremely hot water on the core-ice interface?

By Brian Lacki (not verified) on 10 Sep 2006 #permalink

Theron -

I've sometimes wondered about that as well. You have to remember that our planet is actually quite strange in that we have a huge moon - the earth-moon system is almost a double planet. Hence the planet's axis is very stable and cannot 'process' so that a pole points at the sun (catastrophic for life as we know it..).

So an earth-like single planet (i.e. no large moon) may support very little life.

Now, a planet in orbit around a 'hot jupiter' in the habitable zone would be tidally locked to that planet and therefore have a reasonable, stable day/night cycle. So that could be a more hospitible place for life than a 'standard' earth-like planet. There could, however, be radiation issues..

Andy -

In such a scenario, you would expect to see some mineral exchange at the rock/ice boundary with convection to the ice/water boundary carrying minerals upwards. Such convection happens in the Earth's mantle; I'd almost expect some form of 'vulcanism' as upwelling mineral-rich, warm ice reached the ice/water boundary and melted.

By Andrew Dodds (not verified) on 11 Sep 2006 #permalink


Yeah, the stable axis thing was one thing I was thinking of. Hmm, too few data points to work with right now.

One interesting consideration is the effect of deep oceans on tides. IIRC, on an 'all ocean', uniform Earth, the amplitude of the tides increases with increasing depth. At 27 km (IIRC) the equations describing tides have a singularity, so there would be very large amplitude waves sloshing around the planet, and I suspect it would quickly become tidally locked to the star.

I also wonder if a planet without continents can accumulate an oxygen atmosphere. Sedimentation (which is what keeps photosynthesized organic matter from reoxidizing by burying a small fraction for very long times) would be much less intense without continental erosion.

By Paul Dietz (not verified) on 26 Sep 2006 #permalink

AS far as the planet('s) only being able to sustain life if it was similar to ours I don't completely agree. lets look outside the box a little. What if the lifeforms were completely different. Didn't resemble anything like us at all. (Like maybe DNA or something) But there intelligence level exceeded ours by an enormous amount. (We don't know how DNA does what it does as far as mutations to adapt but hell thats (that referring to the DNA) really smart and we look at it as just blood) So you never know, Even though the planet didn't contain any of the materials or nutrients we need, Or had a high radiation lvl, that does not stop the planet from containing a semi or highly intelligent life form. What if Radiation and Carbon monoxide was what the organisms lived off of ( wee see lifeforms on the bottom of the ocean in environments that we could not imagine living in but they still survive perfectly well) maybe I'm not making sense but thats how I look at it.

Sure, problem is that we don't know what to look for, or whether we'd recognise is it if we saw it.
A lot of astrobio researchers think there is likely more life and weirder life than we appreciate, but until we know more we calibrate our estimates from what we know works