Discovery of a planet around nearby Proxima Cen

Discovery of a planet around nearby Proxima Cen

Today’s big news in astronomy is the discovery of an Earth-mass planet orbiting in the ‘temperate zone’ of Proxima Centauri, aka Alpha Centauri C, by the Pale Red Dot project. What sets this discovery apart from other near-Earth-analogs found in recent years is that the planet is a mere 4.2 light years away and is our Sun’s nearest stellar neighbor. When the Kepler team announced the discovery of Kepler-186f in 2014, an Earth-sized planet found in the habitable zone of an M dwarf in the Kepler Field, by far the number one question we received by both reporters and the general public was ‘Can we go there?’. The answer, unfortunately, was ‘probably not’. The Kepler-186 system is about 500 light years away.
This discovery is exciting for many reasons.

1. We may (in my own words) be able to send a robot there and learn more about the planet.

Breakthrough Starshot is a recently announced new initiative by the Breakthrough Foundation to “demonstrate proof of concept for a new technology, enabling ultra-light unmanned space flight at 20% of the speed of light; and to lay the foundations for a flyby mission to Alpha Centauri within a generation.”

2. It gives support for ACESat, a NASA Ames-proposed mission to search for planets in the Alpha Cen AB binary.

ACESat (of which I was a Co-I and am a huge supporter of) will use direct imaging to look for Earth-like planets in the habitable zones of each star in Alpha Cen AB. While Proxima b is too faint for this method, knowing that a planet formed in this triple star system helps support the science case.

3. This result is inspiring.

Everyone (it seems) gets excited by the prospects of planets in the Alpha Cen system. Centauri Dreams is a beautiful blog that has been around for as long as I can remember, and you can look at any of the posts and comments over the past decade to see the excitement each new milestone has brought to the general public.

4. We need to learn whether planets in the habitable zone of M dwarfs can be habitable. 

Planets around M dwarfs face a LOT of challenges when it comes to habitability. Jill Tarter’s 2007 paper and Scalo’s 2007 paper, while somewhat out of date, discuss the many issues that can help or impede habitability – most of which remain an active area of research and seem to be constantly under debate. A slew of new papers specifically on the habitability of Proxima b, including two that included members of the discovery team (Ribas et al. and Turbet et al.), were posted today.

NASA’s next exoplanet mission – the Transiting Exoplanet Survey Satellite (TESS) will be looking for potentially Earth-like planets to provide targets for NASA’s James Webb telescope, most of them will likely be around M dwarfs. While Kepler has motivated a large number of studies on the topic of planetary habitability around M dwarfs, this Proxima b discovery is sure to spread this topic to an even wider audience — which is good because we need more studies before any planets found by K2 or TESS can be ranked as good targets worthy of valuable James Webb time.

The chances are low (<2%) that Proxima b transits (crosses the face of its star as viewed from Earth), however several groups have been looking. If it does, there would be potential to probe whether it has an atmosphere, and whether it has one suitable to harbor life. Only the (minimum) mass has been measured for Proxima b, because radial velocity techniques don’t provide sizes, so we don’t know for sure that the planet is rocky or Earth-like, though planets of this size have a high probability of being rocky.

The stakes are high just because of the sheer abundance of M dwarfs: more than 70% of all stars are thought to be M dwarfs, and there are hundreds of billions of stars in our galaxy, and there are lots of galaxies in the Universe. Proxima b will be an excellent testbed to learn more about these stellar environments and their effect on planets.

To put Proxima b into context:

Alpha Centauri is a triple star system consisting of a binary star system, Alpha Centauri AB, 4.37 light years away with a third distant companion star Alpha Centauri C (or Proxima Centauri) 4.2 light years from the Sun. The Alpha Cen AB system is the closest binary star system to the Sun and likely the most widely studied star system. Both stars are comparable in size to our Sun and so it has been thought that planets could form in a similar process as they have around the Sun, with only the proximity of the stellar companion to worry about.

One of my first papers as a graduate student investigated planet formation in Alpha Cen AB and showed that Earth-like planets could easily form and remain stable within the stellar habitable zones. We also found here that Alpha Cen B was an excellent target to search for planets. A more modern look at the long-term stability of planets in this system can be found in this paper. I’ve had sort of an obsession with this system ever since. Other studies have questioned whether the building blocks required to form planets could even form given the large stellar perturbations, although more studies are needed to understand the formation of planetesimals in this environment. For a long time Proxima was largely ignored in planet formation studies, as its distance (~15,000 AU from the binary) and small size would have a negligible effect on planets forming around Alpha Cen A or B. It also remains unclear whether it is even bound to Alpha Cen AB.

The Kepler mission has since taught us that Earth-size planets are abundant around M dwarfs and there has been renewed interest in understanding the formation and potential habitability of terrestrial planets orbiting these cool stars. The occurrence rate for small planets in habitable zones around M dwarfs is about 25%, which means that if you look at a sample of 100 stars you could expect there to be 25 Earth-like planets (either 25 stars with a planet, or fewer stars that harbor more than one). So it’s not surprising that an Earth-mass planet exists around Proxima.  What is impressive is that it was detected. The star is quite faint, so detecting a planet as small as Earth is an amazing technical accomplishment.

Just within the past 3 years Earth-size (or Earth-mass) planets have been discovered that span the full range of M dwarf masses. The system of Earth-size planets announced earlier this year (TRAPPIST-bcd) orbit a 0.08 Msun star (the smallest a star can be), and Kepler-186f is an Earth-size planet orbiting a 0.5 Msun star. With the discovery of Proxima b (at 0.12 Msun), we also now know that these potentially Earth-like worlds also exist very close to us, and there’s a real chance that we’ll be able to get a close look at one in the not-too-distant future.

There are surely more exciting discoveries of small planets around small stars to come from the MEarth project (Harvard CfA), the TRAPPIST team (Belgium-led), and other ground-based surveys, as well as from the K2 mission and TESS.

This research is presented in a paper entitled “A terrestrial planet candidate in a temperate orbit around Proxima Centauri”, by G. Anglada-Escudé et al., to appear in the journal Nature on 25 August 2016.

I’ve included some of the beautiful press images from the European Southern Observatory below.

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
This picture combines a view of the southern skies over the ESO 3.6-metre telescope at the La Silla Observatory in Chile with images of the stars Proxima Centauri (lower-right) and the double star Alpha Centauri AB (lower-left) from the NASA/ESA Hubble Space Telescope. Proxima Centauri is the closest star to the Solar System and is orbited by the planet Proxima b, which was discovered using the HARPS instrument on the ESO 3.6-metre telescope.
Credit: Y. Beletsky (LCO)/ESO/ESA/NASA/M. Zamani
This artist’s impression shows 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 between the planet and 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
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.
Proxima Centauri appears as a faint red star towards the lower-right of the picture, a labelled version is available here.
Credit: Digitized Sky Survey 2; Acknowledgement: Davide De Martin/Mahdi Zamani