Into the Planetary Rainforest

by Paul Gilster on December 19, 2011

So-called ‘super-Earths,’ planets larger than the Earth but smaller than Neptune, pose problems to our theories of planet formation. The most recent illustration of this came in the announcement that the candidate planets found by Kepler had now reached 2,326. Remember, many of these will not be confirmed — they’re candidates — but taken in the aggregate, what is interesting here is that one-third to one-half of these candidates fit the super-Earth category. And just as we had a problem with ‘hot Jupiters’ in trying to figure out their orbital position, many of these new planets are likewise in orbits close to their parent star, where the models say they shouldn’t be.

Things seemed so much simpler when we just had a single solar system to worry about, our own. Then, the idea of core accretion could readily account for everything we saw. The dust in the protoplanetary disk was thought to have aggregated into small planetesimals which, in the course of time and numerous collisions, bulked up into planets. It made sense that planets in the inner system would be smaller because the inner part of the disk was thought to have less material for growth, whereas an outer planet would become larger, growing into a gas giant massive enough to pull in a thick atmosphere from the surrounding disk.

As this article by Eric Hand in Nature News points out, the basic model was challenged by finding gas giants in tight orbits, forcing the development of a migration model in which these huge worlds formed out beyond the ‘snow line’ and later made their way into the inner system. Now we have to account for the latest super-Earth findings, as the article points out:

…these models predicted that anything reaching super-Earth size should either become a gas giant or be swallowed by its star, creating a ‘planetary desert’ in this size range. Kepler’s discoveries wreck those predictions. “It’s a tropical rainforest, not a desert,” says Andrew Howard, an astronomer at the University of California, Berkeley. “We hope the theory is going to catch up.”

Hand goes on to talk to Jack Lissauer (NASA Ames), who suggests that some super-Earths could have begun as smaller cores in the outer system that simply never reached the kind of runaway growth that would lead to a Jupiter-class world. Lissauer thinks a planet like this could grow to super-Earth size, and his ideas could explain some of the super-Earths we’ve found with low densities, implying a small rocky core surrounded by a large gas envelope. Even so, we’re still not able to explain denser super-Earths of the kind that are now beginning to be detected.

Kepler’s gradual revelation of smaller and smaller worlds keeps pointing us in the direction of new planet formation models like Norm Murray’s. The astrophysicist (University of Toronto) has tweaked the migration model to have the process of planet formation occur after planetesimals have migrated inward on their own, with the subsequent accretion occurring near the host star. Who knows whether this theory will stand up to the next wave of Kepler results, but if there is one thing we’ve learned from the exoplanet hunt so far, it’s that surprises are abundant, and too doctrinaire a view at this stage is simply asking for revision down the road.

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{ 23 comments }

Abelard Lindsey December 19, 2011 at 14:49

Well, this is what science is about. You make the observations and come up with the models to explain them. If new observations conflict with older models, then obviously those older models are incorrect.

One additional point is that our solar system seems to not be so representative of what we are seeing so far. On the other hand, we still do not have the means to detect planets down to, say, Mars size and, thus, there could be many solar systems like our own that remain to be found.

Mike Walker December 19, 2011 at 20:33

The only people who I’ve seen take a too doctrinaire approach to the theory of planet formation are creationists who, apparently, have a vested interest in there not being any other planets out there that have a chance of harboring life.

Of course, while those ignorant of science regard these puzzles and conundrums as show-stoppers and as evidence that the scientists don’t know what they’re talking about, to planetary sciences, they are merely more data points on the way to a more accurate and comprehensive theory of planet formation.

It’s an interesting process when viewed through the lens of the news media. While scientists are quoted as being “surprised” when unexpected findings occur, surely the biggest surprise of all in the embryonic field of exoplanetary science who be that there were no surprises at all.

Joy December 20, 2011 at 3:07

One interesting thing to note, the delta v to reach even NEO from Earth is pushing the limits of reasonable mass ratios for chemical rocketry, even with staging expendable boosters. (Most other propulsion methods are useless for getting off the planet, as they cannot produce the required > 1g thrust) If superearths are much more common than ~1 earth mass planets … well then any ETs living on those have little hope of launching even a Sputnik.

Alex Tolley December 20, 2011 at 9:45

@Joy
Or perhaps due to necessity, ETs will develop nuclear rockets to reach orbit?

Ole Burde December 20, 2011 at 16:23

If multicellular life exists on a “super earth” , it would probably be restricted to the sea. Even insects would collapse under 100 Gs, and bigger mobile land-life forms would be necesary for intelligence. Being therefore restricted to the sea, it would be very difficult and perhaps impossible for an intelligent species (like earths dolfins) to achieve any kind of break-through useful toolmaking capabilities. This might be one of the major reasons for SETI not hearing any radiosignals..

Rob Henry December 20, 2011 at 19:05

Joy and Alex Tolley, it strikes me that on a superearth with similar atmospheric composition to ours, we would also have the alternative of awaiting air-breathing rocket technology to attain our equivalent to Sputnik. With multiple options to get them to low orbit our real concern should be whether they develop the right mindset along their path of technological development.

Our current understanding is that the expected surface atmospheric density of a planet will tend to increase faster than gravity if one was plotted against the other. This would make powered flight easier, not harder, on the typical superearth than the typical earth. Their Wright Brothers would have it easier, and the role of their Montgolfier Brothers a cinch.

Of cause all this does not address Joy’s second concern of whether they will ever leave a low superearth orbit.

Duncan Ivry December 20, 2011 at 19:22

Ole Burde: “If multicellular life exists on a “super earth” , it would probably be restricted to the sea. … it would be very difficult and perhaps impossible for an intelligent species (like earths dolfins) to achieve any kind of break-through useful toolmaking capabilities.”

Let us assume, “they” do not swim through the see but walk on the bottom of shallow water areas (like “we” don’t fly like birds but walk on the ground). They would start with stone tools like we did — the Next Major Hand Axe Physics Breakthrough (TM) will happen. They have fire too, e.g. from hot smokers. They would see something interesting above the water, may be even celestial bodies. Perhaps they would be interested in astronomy. This would be utterly nice, wouldn’t it?

Phil December 20, 2011 at 20:10

Maybe exoplanets in that still undiscovered 0.3>1 Earth mass realm might be better candidates for tool making civilisations? (provided all other habitability requirements are met of course). The fact that these will be feindishly difficult to detect poses a fascinating challenge for the next decades.

P

Eniac December 20, 2011 at 23:40

Ole:

Even insects would collapse under 100 Gs, and bigger mobile land-life forms would be necesary for intelligence.

I don’t think either of these is true.

100 g could be as easily supported by an animal of several centimeters in height as we (or T-Rex) support one g. The relationship between size and maximum supportable gravity is inverse linear.

The correlation between body size and intelligence is weak, and AFAIK there is no compelling reason that a centimeter sized animal (like a mouse) could not be as smart as we are.

Ronald December 21, 2011 at 6:13

This great news just in: Kepler has discovered its first two earth-sized planets (though not nearly in the HZ):
http://www.nature.com/news/kepler-discovers-first-earth-sized-exoplanets-1.9688
http://www.nature.com/nature/journal/vnfv/ncurrent/full/nature10780.html

djlactin December 21, 2011 at 7:09

Given:
1) that a planet in an earthlike orbit would need ~1 y to circle its star,
2) that 3 orbits are required before the planet’s presence could be confirmed, and
3) that Kepler went up in March 2009,
I expect that announcements of such planets will begin in late 2012/early 2013.
Wagers?

Ronald December 21, 2011 at 9:25

Abelard Lindsey rightly points out that the fraction of smaller planets is also going up continuously, according to the latest Kepler data the fraction of roughly earth-sized planets is about 10% and growing.

I think an interesting question concerning the super-earths is whether they are indeed super-earths, i.e. accreted as such, ot the remnants of Neptune class subgiants or even of true gas giants, whose gas and liquid envelope has eroded, blown away, as a result of their close proximity to their host star.

It remains to be seen whether these super-earths are also common, or even present at all, at greater orbital distances.

Mike Walker says it all, great comment.

BTW, the gravity on a superearth of 5 Me and earthlike density would only be 1.7 times earth gravity and on a 10 Me superearth of (similar density) almost 2.2 times earth gravity. Fot a 3 Me planet it would only be 1.4 times. So gravity would indeed be somewhat greater, but not that much, not so much that life could not be adapted to it. The planet being an ocean planet would of course be an entirely different matter.

Ronald December 21, 2011 at 9:34

A bit further to Abelard Lindsey’s and Mike Walker’s excellent comments, I cannot help but noting that in nature and science there are no real problems, only natural phenomena and (better) explanations for those.

Problems only exist for those people that dearly wish to see their own dogma’s confirmed, be it for religious/philosophical or for career reasons.

Ronald December 21, 2011 at 9:43

Ole: almost redundant by now (I hope), but: 100g would require a super- Jupiter giant planet with (much) greater than earth density. I doubt that planets with such gravity even exist.

Ole Burde December 21, 2011 at 14:50

Ronald
Thanks for the correction . Gravity on a super earth would probably be between 1.5 and 5 times what it is on earth . It seems that problems DO exist for people for people who forgot most of their highscool physics !

Eniac
” The correlation between body size and intelligence is weak, and AFAIK there is no compelling reason that a centimeter sized animal (like a mouse) could not be as smart as we are.”
The correlation IS weak , but its the only real evidence we have . On earth the only species that can claim the title are mostly above the 30 kg size .
Many much smaller species are capable of specific types of problem solving which normally exist in their environment , but only the bigger ones are capable of extrapolating new solutions from known problems .
No one has ever sucseeded in explaining what consiousnes is , but if it is connected to a higher lewel of general intelligence , then the combined proces might demand a big quantity of brain volume .

Rob Henry December 21, 2011 at 16:32

Ole your remarkable. How did you so effortlessly determine the intelligence differences between bees that have a symbolic language and can count, with that of bigger mammals who do not posses these attributes but whose intelligence is so different in nature that I would have difficulty evaluating that apian comparison.

Eniac December 21, 2011 at 22:59

Ole:

The correlation IS weak , but its the only real evidence we have

The weakness of the evidence is hardly a good justification for the sweeping conclusion you appear to have drawn from it.

Shaun Steenkamp December 22, 2011 at 0:10

I may be wrong here, but wouldn’t it be more logical to assume that there would be more material available closer to the forming star for large planet formation than further away from the star?

If you imagine a large stellar nursery I would have thought that the star would pull large amounts of material towards itself within its gravitational influence. This would result in a greater density of material around the star. This density would slowly decline the further you move from the star and then return to the average density of the surrounding interstellar material (whether that is higher or lower than the density at the edge of the gravitational influence of the star). The decline in density is due to the waning gravitational influence of the star. A comparison could be drawn to the Milky Way galaxy which has a higher density of stars around its core and a lower density around further away.

Under this assumption you would expect larger planets close to the parent star and smaller ones further out.

I quite like Ronald’s suggestion that large gaseous planets form around the star initially (such as Neptune). However, after the star has formed and the remaining material in the system is ejected solar winds in the near vicinity of the star result in the evaporation of the dense gaseous atmospheres.

Ole Burde December 22, 2011 at 12:23

Eniac
“The correlation IS weak , but its the only real evidence we have

The weakness of the evidence is hardly a good justification for the sweeping conclusion you appear to have drawn from it.”

If you try to relate to the worl in a statistical way , then what you dislike as as a “sweeping conclution” becomes just a working-hypotesis , which at the present time seems to have a higher probability of beeing true than other alternative theories .
Some people , like myself ,feel the need to have such a working hypotesis . It helps when you have to solve problems without the time to evaluate all possible solutions . If you were forced to decide where to invest an effort in searching for intelligence , where would you look FIRST ? Among Bees ?
Come On !

Ronald December 23, 2011 at 6:45

@Shaun Steenkamp: I think the general idea among astronomers with regard to (giant) planet formation is that the *total* amount of available primordial dust increases with distance (up to a maximum after which it declines again), simply because the total orbit of an embryonic planet sweeping up this material logically increases with distance.

And honestly, the idea of these super-earths being remnants of close-in ice and gas giants of which the gaseous envelopes have been blown away, is not originally mine (I think, if it is though, I hereby want to claim this as a new theory ;-) ).
This idea will be tested as more and more Kepler data come in from greater orbital (AU) distances: are these super-rocky planets also present at greater distances from the host star (beyond any ‘snow-line’ or the like) or are only ice and gas giants found at those greater distances? That remains to be seen.

Ronald December 23, 2011 at 6:52

Further to my previous comment to Shaun: maybe it is even rather irrelevant whether the super-earths are remnants of ice and gas giants whose gaseous/liquid envelopes have been blown away, or that those envelopes did not, could not, form in the first place. The result is the same: only a large rocky core, without the gaseous/liquid envelope.
Again, it would be very interesting and testing to find out whether these super-earths also exist at greater distances.
See also CD’s post after this one: ‘Kepler Finds Earth-sized Planets’.

Eniac December 24, 2011 at 0:37

If you were forced to decide where to invest an effort in searching for intelligence , where would you look FIRST ? Among Bees ?

If we had a choice between monkeys and bees: monkeys, definitely. But we do not have such a choice. We have nothing.

I objected to very unambiguous statements (“insects would collapse” and “bigger mobile land-life forms would be necessary for intelligence”) which I think are both false. I am not against making prudent choices of were to look, but surely we should not base our decisions on false assumptions.

Rob Henry December 25, 2011 at 5:54

“If you were forced to decide where to invest an effort in searching for intelligence , where would you look FIRST ? Among Bees ?”

I have a very different take on this statement than Eniac. For Ole’s reference to be maximally relevant it should be made in regard to ape and bee analogues in alien biosystems.

In one biosystem that contains both, I would also look first to the apes, but just for these two reasons
1) I would transfer my Earthly biases to this new system as a sort of default setting, and on Earth I also suspect that it is true
2) If these alien bees were really intelligent, the likely hive nature of that intelligence would mean that contact with them might be superlatively difficult.

If the apes and bees looked like the highest creatures in their respective worlds, then I would assume it likely that each is filling the niche of most complex creature on their home world, and that, because of this, it is quite possible that the selective pressure for intelligence is abnormally high in each case. In this new scenario I would only be swayed by the flexibility with which the tool use and constructions of these species change over changing environments.

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