We’ve been paying a lot of attention to Centauri A and B in the past two years, but what about Proxima Centauri? After all, this is the closest star to our Sun, a fifth of a light year out from the two major Centauri stars, and free of the close binary problem. You would think this small red dwarf would rank higher on our list of astrobiologically interesting places, but until recently, that red dwarf status has been an encumbrance. It has been only within the last eleven years that the presumed tidal locking of planets in the habitable zone of such stars has been found not to be a necessary deterrent to the formation of a stable climate.
Today, M dwarf interest grows. There’s at least the chance of a workable ecosystem around such a star, assuming flare activity (common to these stars) might act more as an evolutionary stimulus than a deterrent to life. Moreover, the long lifetimes granted to M dwarfs mean that stable environments could exist for many billions — perhaps hundreds of billions — of years. This is why we’ve seen a recent florescence of M dwarf studies, with a keen interest in their astrobiological prospects, and why Proxima Centauri remains an interesting target. And although it hasn’t gotten the press of its larger siblings, Proxima has generated studies that are closing in on characterizing its system.
Image: Alpha Centauri, with components Centauri A and B doctored for clarity. The arrow marks Proxima Centauri. Credit: European Southern Observatory.
We can already say this about Proxima planets: If they exist, they are no larger than 0.8 Jupiter masses in the range of orbital periods ranging from one to 600 days. That’s from radial velocity studies published in the late 1990s. This work is now complemented by seven years of high precision radial velocity data gathered with the UVES spectrograph at the European Southern Observatory. Michael Endl (McDonald Observatory) and Martin Kürster (Max-Planck-Institut für Astronomie) address the question of what kind of planets we can exclude from the habitable zone of Proxima Centauri based upon these data.
Proxima’s habitable zone, remember, is in close because this is a small star — the authors assume 0.12 solar masses, a reasonable estimate if on the high side, for reasons they explain in their paper. The habitable zone then becomes 0.022 to 0.054 AU, which corresponds to an orbital period ranging from 3.6 to 13.8 days. And the UVES data make it clear that no planet of Neptune mass or larger exists out to a distance of 1 AU.
For periods of less than 100 days, no super-Earths are detected larger than about 8.5 Earth masses. And for the actual habitable zone of Proxima Centauri we can rule out planets larger than 2-3 Earth masses in circular orbits. Needless to say, this doesn’t rule out planets of Earth mass or smaller in this zone.
The sensitivity of these studies is only improving:
With the results from this paper we demonstrate that the discovery of m sin i ≈ 1 M⊕ [one Earth mass] is within our grasp. Since sensitivity is a function of RV [radial velocity] precision, number of measurements and sampling, adding more points to the existing data string in a pseudo-random fashion, will allow us to improve the detection sensitivity over time.
Do note the above qualifier: We can rule out Proxima planets of 2-3 Earth masses in circular orbits. This is one of several limitations on the study:
Limits for planets on eccentric orbits are typically slightly higher… Planets with masses above our mass threshold for circular orbits can still exist around Proxima Cen on eccentric orbits. We also considered only the case of a single planet. The RV signals of a multi-planet system with several low-mass bodies, [are] likely to be more difficult to detect by a pure periodogram analysis (depending on their period spacing and mass ratios) and require signiﬁcantly larger data sets. Simulations to determine the mass limits for multiple planets [are] beyond the scope of this paper.
Thus Proxima Centauri remains a prime target, if one for which we still have no discovered planets. We are moving ever closer to the ability to detect Earth mass planets inside the habitable zone around this star, a capability we’ll hope to refine still more in coming months. After all, a Mars-size planet in the right place could provider an abode for life even if beneath our current threshold for detection. The paper is Endl and Kürster, “Toward detection of terrestrial planets in the habitable zone of our closest neighbor: Proxima Centauri,” accepted for publication in Astronomy & Astrophysics (available online).