We’ve recently discussed Greg Laughlin and Jeremy Wertheimer’s work on the possible role of Proxima Centauri in destabilizing the Centauri A and B debris disk and bringing volatiles to the inner system. Our deepening knowledge of the Centauri system is one of the most energizing aspects of the exoplanet hunt, for its proximity inexorably makes Alpha Centauri of high astrobiological interest.
And no one has done more significant work on planet formation around binary stars than Elisa Quintana and Jack Lissauer (NASA Ames). The two have examined the possibilities of terrestrial worlds around Centauri A and B and are continuing with the study of other binary scenarios. Now they have extended their analysis to binary systems whose stars are much closer to each other than Centauri A and B.
Their new paper is significant for planet hunters because more than half of all main sequence stars are in binary or multiple systems, whereas our basic models for planet formation have been based on single stars. That conventional model relies on the accretion process inside a disk of dust and gas that surrounds the newborn star. We’re now finding disks of such materials around binaries, with the mass of the disks comparable to those found around single stars, giving credence to the notion that planet formation in binary systems is not unusual.
Quintana and Lissauer’s paper reports on their simulations of the late stages of terrestrial planet formation in stars with separations between 0.05 AU and 0.4 AU, assuming a total mass of 1 solar mass for the two stars. The duo worked with fourteen different short-period binary combinations, varying the initial orbits of the stars and their eccentricities and factoring in the effects of gas giant planets on the model of Jupiter and Saturn.
The scientists used the same initial disk parameters they used to simulate planet formation around the individual Centauri stars in their 2002 paper (see below), and compared their results to planet formation simulations using the Sun, Jupiter and Saturn that assume the same initial protoplanetary disk. The result: The planetary systems forming around binaries less than 0.2 AU apart turn out to be similar to those forming around single stars, while fewer planets form around systems with wider spacing. At 0.3 AU, the accreting disk is perturbed enough to make the formation of terrestrial worlds near 1 AU unlikely, although even in the wider spacings smaller terrestrial worlds (think Mercury) do emerge.
Centauri Dreams‘ take: We haven’t yet detected exoplanets orbiting both members of a main sequence binary system, but this is likely the result of our methods — Quintana and Lissauer note that short-period binaries are not included in radial velocity search programs because of their complicated spectra. Such planets are doubtless out there, and their parent stars may play an interesting role in planet hunting. From the paper:
An additional beneﬁt of understanding the differences between planet formation around single stars and that around close binaries is that for eclipsing binaries, the contrast ratio between brightness of the stars and that of the planet(s) is reduced during the eclipse. For a total eclipse of identical stars, this reduction is a factor of two; as lower mass main sequence stars can be just slightly smaller but signiﬁcantly less luminous, the detectability of the planet can be enhanced by more than a factor of two when the fainter star transits the brighter one. In an evolved close binary having undergone mass transfer, the fainter star can actually completely eclipse its much brighter companion, leading to an even larger improvement in planetary detectability.
The paper is Quintana and Lissauer, “Terrestrial Planet Formation Surrounding Close Binary Stars,” now accepted for publication in Icarus and available here. The scientists have completed a second set of simulations on wide binaries and are writing up the results. I’ll have a summary of those findings soon, and at some point in the near future, I want to return to the 2002 paper “Terrestrial Planet Formation in the Alpha Centauri System” (available here as a PDF) a key study that has changed our thinking on terrestrial worlds and habitability around the nearest stars.