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Planetary Prospects Around Proxima

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.

Alpha Centauri stars including Proxima

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).

Comments on this entry are closed.

  • andy July 15, 2008, 14:05

    To be pedantic, the 0.8 Jupiter mass limit is from astrometry, which means it is a limit on true masses. This latest work uses the radial velocity method, which puts limits on the quantity of mass times the sine of the inclination m sin(i): more massive planets could exist in low-inclination (nearly face-on) orbits, which have low sin(i).

  • Ronald July 16, 2008, 5:38

    When it comes to red dwarfs (M stars) I can think of better candidates in the stellar neighborhood.

    For instance Lalande 21185, only 8.3 ly away, which is considerably bigger and brighter (M2V, 46% solar mass, about 1% solar total brightness (including infrared) and reasonably high metallicity at about 60% of solar).

    It is know to have a planetary system, at least 3 Jupiter class planets at about 2, 10 and over 11 AU. Its habitable zone would be at about 0.22 AU (accounting for infrared).

  • Ronald July 16, 2008, 5:40

    Adding to my previous post:
    OK, Lalande 21185 is also known to be a flare star, I do not know how seriously, but that will gradually go away as Lalande 21185 grows up ;-)

  • David July 16, 2008, 8:03

    This is where the question of whether Proxima Centauri
    is bound to Alpha Centauri A/B becomes relevent.
    This is because if it is, then it implies a common origin for all three bodies and so Proxima Centauri also has a metallicity about three times that of the Sun. So we have an example of a very metal-rich star which not only has no hot-jupiters, but possibly no massive terrestrial-type planets which gives another wrinkle to the whole issue of stellar metallicity and presence of exoplanets.

    The complicated spectra of M dwarfs makes it hard to calculate their metallicity so there has been some work in looking for M dwarfs orbiting other stars where
    the metallicity is easier to find.

  • Ronald July 16, 2008, 10:36

    @David: this brings the issue of the origin of binary star systems.

    I understand that by far the most binary (and multiple) stars have originated in situ, that is during star formation from the same primordial gas cloud.

    And only a very small fraction of binaries etc. are thought to have resulted from the very rare event of gravitational capture, i.e. having become gravitationally bound in an eternal dance as a result of a (too) close encounter during their cosmic journeys.

    The latter situation could possibly be concluded from a very different chemical composition (metallicity) and age of the component stars.

    I wonder whether sheer distance might also be indicative of different origin and gravitational capture: Proxima is so distant from Alpha Centauri A and B, that it seems quite unlikely that they would have formed together. In that case, Proxima may well have a very different metallicity (and age) from the other two.

    BTW: I also wonder whether a similar situation (different origin and gravitational capture) might be the case for Zeta Reticuli 1 and 2, which are some 9000 AU apart.

  • devicerandom July 16, 2008, 11:58

    After all, a Mars-size planet in the right place could provider an abode for life even if beneath our current threshold for detection.

    Chances of a Mars-sized planet hosting life are smaller than for a Earth or super-Earth. The gravity would be too low to host a significant atmosphere (and possibly liquid water would photodissociate and hydrogen evaporate in space). Also, plate tectonics would grind to a halt quite soon. It is possible that both conditions are not strictly needed, however it can be argued it is a far less hospitable place than an Earth-sized object.

  • David July 17, 2008, 8:52

    Ronald,

    There has been some recent work by Greg Laughlin and one of his students that show that Proxima Centauri probably is bound to Alpha Centauri A/B. The published radial velocities of Proxima Centauri are a couple of decades old and are not quite precise to be sure. The Endl and Kurster data would be more than sufficient this purpose, maybe they will use it for another paper.

  • Ronald July 18, 2008, 3:32

    @David:
    thanks, I know, but my question is not so much whether these very wide binaries/multiples are gravitationally bound (as binaries/multiples they are per definition), but what their origin is: formed together or later captured by gravity.

  • Luc August 1, 2008, 5:14

    Seems to be a more optimistic paper about close (about 4.4 LY or so) planets than a previous one by
    Thébault et al discussed here a few days ago by Paul (14th July 08).
    Cheers
    L