A planet in the habitable zone around Proxima Centauri? The prospect dazzles the imagination, but then, I’ve been thinking about just that kind of planet for most of my life. Proxima Centauri is, after all, the closest star to our own, about 15000 AU from the primary Alpha Centauri stars (though thought to be moving with that system). A dim red dwarf, Proxima wasn’t discovered until 1915, but it quickly seized the imagination of science fiction writers who pondered what might exist around such a star. Murray Leinster’s story “Proxima Centauri” (1935) is a clanking, thudding tale but it still evokes a bit of the magic of one of the earliest fictional interstellar voyages.
Image: This wide-field image shows the Milky Way stretching across the southern sky. The beautiful Carina Nebula (NGC 3372) is seen at the right of the image glowing in red. It is within this spiral arm of our Milky Way that the bright star cluster NGC 3603 resides. At the centre of the image is the constellation of Crux (The Southern Cross). The bright yellow/white star at the left of the image is Alpha Centauri, in fact a system of three stars, at a distance of about 4.4 light-years from Earth. The star Alpha Centauri C, Proxima Centauri, is the closest star to the Solar System. Credit: A. Fujii.
More recently, Stephen Baxter pretty much nailed Proxima Centauri b in his depiction of a just over one Earth-mass planet in the habitable zone called Per Ardua — this was in Baxter’s 2015 novel Proxima. Baxter’s planet was at 0.04 AU, and a little more massive than Earth; the real thing is 0.05 AU and 1.3 Earth masses. I would call that very nice work. Baxter has also noted, with considerable justification, that if we find a truly habitable planet in the very next system to our own, the implication is that such planets are quite common.
Image: This image of the sky around the bright star Alpha Centauri AB also shows the much fainter red dwarf star, Proxima Centauri, the closest star to the Solar System. The picture was created from pictures forming part of the Digitized Sky Survey 2. The blue halo around Alpha Centauri AB is an artifact of the photographic process, the star is really pale yellow in colour like the Sun. Credit: Digitized Sky Survey 2. Acknowledgement: Davide De Martin/Mahdi Zamani.
Having been at the Breakthrough Starshot meetings all this week, I’m delighted to see that we now have a potential destination; i.e. an actual rather than assumed planet around one of the stars in the system nearest to us. Finding Proxima’s planet has been a long process, drilling down to the kind of measurements that can reveal its presence. Up until now we’ve been excluding larger planets in various kinds of orbits around Proxima, but the prospect of something Earth-sized in the habitable zone remained open. I hasten to add that Breakthrough Starshot has made no decisions about its target at this point, but it’s clear that Proxima b is going to be a prime contender.
I’m going to let Guillem Anglada-Escudé, head of the Pale Red Dot project, and his collaborators describe what his team has found. Noting that uneven sampling and the longer-term variability of the star are reasons why the signal could not be confirmed from the earlier data, the researchers go on to describe these key characteristics of the planet. From the paper:
The Doppler semi-amplitude of Proxima b (? 1.4 ms?1) is not particularly small compared to other reported planet candidates. The uneven and sparse sampling combined with longer-term variability of the star seem to be the reasons why the signal could not be unambiguously confirmed with pre-2016 rather than the amount of data accumulated.
And here’s what we’ve been waiting to hear:
The corresponding minimum planet mass is ? 1.3 M? . With a semi-major axis of ?0.05 AU, it lies squarely in the center of the classical habitable zone for Proxima. As mentioned earlier, the presence of another super-Earth mass planet cannot yet be ruled out at longer orbital periods and Doppler semi-amplitudes <3 ms ?1 . By numerical integration of some putative orbits, we verified that the presence of such an additional planet would not compromise the orbital stability of Proxima b.
Image: Guillem Anglada-Escudé, head of the Pale Red Dot project and lead author of the paper on the discovery of Proxima Centauri b.
And there we are, our first assessment of a planetary system around Proxima Centauri. The team’s analysis taps into previous Doppler measurements of Proxima Centauri coupled with the follow-up Pale Red Dot campaign of 2016. The Doppler data draws on the HARPS (High Accuracy Radial velocity Planet Searcher) spectrometer and UVES (the Ultraviolet and Visual Echelle Spectrograph). The search methods and signal assessment are thoroughly discussed in the paper (citation below). Key to the effort was what Anglada-Escudé and team call “[a] well isolated peak at ?11.2 days” that appeared in the pre-2016 Doppler data. The HARPS Pale Red Dot campaign was created to confirm or refute this 11.2-day signal. And confirm it they did.
Image: This artist’s impression shows a view of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image to the upper-right of Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface. Credit: ESO/M. Kornmesser.
We have a long way to go before knowing whether a planet around a red dwarf like this can truly be habitable. Tidal locking is always an issue because a planet this close to its host (Proxima Centauri b is on an 11.2-day orbit) is probably going to have one side fixed facing the star, the other in permanent night. There are papers arguing, however, that tidal lock does not prevent a stable atmosphere with global circulation and heat distribution from occurring.
And what about Proxima’s magnetic field? The average global magnetic flux is high compared to the Sun’s (600±150 Gauss vs. the Sun’s 1 G). Couple this with flare activity and there are scenarios where a planet gradually has its atmosphere stripped away. A strong planetary magnetic field could, however, prevent this erosion. Nor would X-rays (400 times the flux the Earth receives) necessarily destroy the planet’s ability to keep an atmosphere.
Image: An angular size comparison of how Proxima will appear in the sky seen from Proxima b, compared to how the Sun appears in our sky on Earth. Proxima is much smaller than the Sun, but Proxima b lies very close to its star. Credit: ESO/G. Coleman.
And then there’s the matter of the planet’s origins, and how that could affect what is found there. From the paper:
…forming Proxima b from in-situ disk material is implausible because disk models for small stars would contain less than 1 M Earth of solids within the central AU. Instead, either 1) the planet migrated in via type I migration, 2) planetary embryos migrated in and coalesced at the current planet’s orbit, or 3) pebbles/small planetesimals migrated via aerodynamic drag and later coagulated into a larger body. While migrated planets and embryos originating beyond the ice-line would be volatile rich, pebble migration would produce much drier worlds.
We can now hope for further data on Proxima Centauri b through transit searches, direct imaging and further spectroscopy. Ultimately, of course, we can think about the prospects of robotic exploration, the sort of thing we’ve been discussing here on Centauri Dreams for the last twelve years. No star is closer, and few will reward follow-up study more than this one. I need to get into a meeting and will have to let that wrap this up, but you can be sure there will be a lot more to say about Proxima and the entire Alpha Centauri system as the analysis continues.
The paper is Anglada-Escudé et al., “A terrestrial planet candidate in a temperate orbit around Proxima Centauri,” Nature 536 (25 August 2016), 437-440 (abstract).