The discovery of Centauri B b, a small planet with a mass similar to Earth, continues to percolate in the news even if the initial buzz of discovery has worn off. Science News gives the new world a look in a recent article, noting the fact that with an orbital period of 3.236 days, this is not a place even remotely likely for life. Surface temperatures in the range of 1200 degrees Celsius are formidable obstacles, but of course the good news is the potential for other planets around Centauri B and, indeed, around its larger companion.
Centauri A may well host interesting worlds, but it’s a tough study because it’s given to the kind of stellar activity that can more readily mask a planetary signature than the quieter Centauri B. Even so, we can imagine the possibility of two planetary systems in close proximity, a scenario that would surely propel any technological civilization around one to investigate the other. We don’t have the driver for spaceflight in our system that an Earth-like world around Centauri B might have, a second habitable planet breathtakingly close around another star.
If we’ve ruled out planets larger than Neptune around any of the three Alpha Centauri stars, that leaves the door open for the small worlds that could be the most interesting if one or more turned up in the habitable zone, and it’s worth noting that on this score, Proxima Centauri is still in the game. But right now the incredibly tricky detection of Centauri B b needs confirmation, which could be delivered by Debra Fischer (Yale University). You’ll recall that Fischer has been working at Cerro Tololo (CTIO) in Chile to develop the high-resolution spectrometer known as CHIRON, commissioned in March of 2011, as part of her team’s search for rocky Alpha Centauri planets.
Leaving Copernicus Behind
Centauri B b might also be confirmed through detection of a transit, the chances being estimated in the region of 10% and perhaps, according to Greg Laughlin (UC-Santa Cruz) as high as 25%. While the necessary work continues, let’s move beyond the Alpha Centauri stars for a moment to talk about Laughlin’s latest work with Eugene Chiang. Exoplanet hunters have learned through experience to question assumptions, the most obvious of which is that our Solar System is in some sense ‘normal.’ Or as Laughlin writes on systemic, “There is an intriguing, seemingly anti-Copernican disconnect between the solar system and the extrasolar planets.”
Image: Exoplanet hunter Greg Laughlin, whose latest work re-examines how super-Earths form close to their stars. Credit: UC-Santa Cruz.
Maybe the reason exoplanets so often surprise us is that we base our thinking on our own Solar System, and the minimum-mass Solar nebula from which it grew, considering this a template. The rest of the galaxy may have other ideas. Consider that close-in super-Earths are common. Planets like these, showing up in abundance in Kepler data and Doppler velocity surveys, are a challenge to explain. Laughlin and Chiang say that more than half, if not nearly all Sun-like stars have planets with radii between 2 and 5 times that of Earth and orbital periods of less than 100 days. The researchers write:
Super-Earths are not anomalous; they are the rule that our Solar System breaks. In a sense, the burden of explaining planetary system architectures rests more heavily on the Solar System than on the rest of the Galaxy’s planet population at large.
The problem, then, is that our Solar System has no planets inside Mercury’s 88-day orbit. Is it possible our Solar System did not undergo the same kind of formation history that may be the dominant mode in the galaxy? To explore this, the researchers look at migration issues, for it is commonly thought that short-period planets formed several AU out from their stars and then migrated to their present location. But disk migration is poorly understood, and while it may be necessary to explain hot Jupiters, Laughlin and Chiang say it may not be the mechanism to explain the majority of planetary systems with super-Earths in inner orbits.
The alternative: Forget orbital migration and consider the possibility that super-Earths form right where they are, in circumstellar disks that extend inward from 0.5 AU. The researchers go to work on constructing a new template, the Minimum Mass Extrasolar Nebula (MMEN), which allows them to explore how such planets could form near their star:
Our order-of-magnitude sketches in this regard are promising. In-situ formation at small stellocentric distances has all the advantages that in-situ formation at large stellocentric distances does not: large surface densities, short dynamical times, and the deep gravity well of a parent star that keeps its planetary progeny in place.
The basic properties of planets forming at their current orbital distance are made clear:
In-situ formation with no large-scale migration generates short-period planets with a lot of rock and metal and very little water. The accretion of nebular gas onto protoplanetary cores of metal produces H/He-rich atmospheres of possibly subsolar metallicity that expand planets to their observed radii. Retainment of primordial gas envelopes against photoevaporation leads to planets that can be similar in bulk density to Uranus and Neptune while being markedly different in composition. Close-in planets are not water worlds.
Laughlin and Chiang believe there should be observational consequences to such predictions, making the theory falsifiable. Hot young stars should lack close-in super-Earths because they would be too hot for planetesimal-building dust to survive. Brown dwarfs and M dwarfs should have close-in super-Earths and Earths orbiting them. Close-in planets grown from close circumstellar disks should also have orbital planes aligned with the equatorial planes of their host stars. I’ll send you to the paper to go through the entire list of predictions, all of which should allow these ideas to be probed, but I do want to mention one last prediction having a bearing on Centauri B b. For if Laughlin and Chiang are right, then binary systems offer a good test.
After all, close binary systems should make planetary migration extremely difficult. The close companion would disrupt planet formation at large distances from the star. Planets orbiting Centauri B inside the 0.5 AU boundary would be incompatible with migration, and of course, we now have such a planet, along with the likelihood of finding more. Here I drop back to the Science News article, which quotes Laughlin on Centauri B: “I think that the odds that there’s an interesting planet, a truly interesting planet in the system, are very high, given that this one is there.” And if he’s right, that interesting, potentially habitable world may serve as further evidence for the theory that such planets formed right where they are found.
The paper is Chiang and Laughlin, “The Minimum-Mass Extrasolar Nebula: In-Situ Formation of Close-In Super-Earths,” submitted to Monthly Notices of the Royal Astronomical Society (preprint).