Discussion and news about the modern effort to understand the nature of life on Earth, finding planets around other stars, and the search for life elsewhere in the universe

Sunday, May 22, 2011


Busy week, the week that was. Catch me on NPR's All Things Considered offering up some thoughts on the current SETI situation. Also my posts have been slowed as I get into a final stretch on a new book project for a general audience. If you like astronomy, cosmology, black holes and astrobiology you're in for a treat - I hope. Hitting the stores in 2012, I'll be saying more about this over time.

As for right now, well.....

There's a certain poetry to the astronomical news that's been hitting the media about planets that may be doing precisely as their name implies (πλανήτης in case your Greek isn't as rusty as mine). Painstaking monitoring of about 50 million stars in our galaxy by the MOA and OGLE gravitational microlensing surveys have revealed a very substantial population of Jupiter-sized objects that orbit at least 10 times further from their stars than the Earth does the Sun, and may not even be bound to stars at all. Sumi et al. report these results in Nature, along with a nice commentary by Wambsganss.

Although only ten such candidates are actually detected the statistical implication (since lensing events are so incredibly rare) is that there are twice as many of these planetary bodies than normal main-sequence (hydrogen burning) stars in our main galactic terrain. That is a lot. Even more interestingly Sumi et al. claim that most of these objects, perhaps 75%, may be unbound from any parent stellar systems and are true wanderers. The basis for this argument comes from existing constraints on long orbit exoplanets. These are obtained from projects trying to directly image such worlds. Bottom line is that the imaging efforts around stars do not see as many planets as the microlensing results would imply, hence these objects have gotta be out in interstellar space.

Personally I think there is every reason to believe that there really is a huge population of free-floating planets out there. A couple of years ago various researchers came up with a framework for explaining exoplanetary orbital architectures that requires episodes of intense planet-planet gravitational interaction within a system. One consequence; lots of planets ejected away from their birth places. My colleague Kristen Menou and I dabbled in this to investigate the predictions for planet imaging surveys. It was fun. Many planetary systems may be born in configurations that are inherently dynamically unstable, chaotic. Flinging worlds to the void is a great way to 'cool' the system down.

So, apart from the wow-factor of rogue/free/unbound planets, there is real reason to chase and confirm this result. It could be a pivotal clue that tells us not only how most planetary system achieve their configurations, but also confirms that planet formation itself is an efficient process. You have to make planets a plenty around stars in order for this to all work. The alternative is also fascinating - perhaps these worlds form via a route more akin to that of stars and brown-dwarfs, another feather in the cap of gravitational accretion.


Mark Piffer said...

Are these wanderers mainly floating around the galactic centre like stars or are their trajectories more chaotic due to their ejection process from a stellar system? If the latter (and maybe also if not), wouldn't their presence have consequences on star forming, as, by their sheer number, every gas cloud is being perturbed sometime during its existence by such a "gravitational seed"?

Caleb Scharf said...

That's a great question. I think the ejection velocities (based on what I've seen in my own simulations of dynamical cooling in planetary simulations) are likely to be typically quite low - certainly not enough to add much to the typical velocity with which the stars themselves are buzzing around in the Galaxy. They will therefore share a major component of the motion of their original star system as it moves in orbit around the Galaxy. The question of what these ejected worlds might mean within a star/planet forming nebula is really an excellent one. I'm not sure off the top of my head what the effect might be. Obviously even a big Jupiter-like world is still a lot less massive than even an M-dwarf star and even a 'thick' proto-stellar nebula is still pretty tenuous, so they might have relatively little to do with each other. On the other hand, a close encounter of one of these free ranging planets with a proto-planetary disk at a critical stage could presumably perturb it. Definitely worth further thought.

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