Intensifying the Proxima Centauri Planet Hunt

By Paul Gilster, author of Centauri Dreams

There will always be a ‘proxima’—a star that is closest to our own—but it won’t always be Proxima Centauri, which in tens of thousands of years will doubtless revert to a different name, perhaps Alpha Centauri C or some other designation. We live in a dynamical universe, one in which the red dwarf Ross 248 will (in forty thousand years or so) be the new ‘proxima.’ We can even anticipate stars being much closer than Proxima Centauri is today. Go 1.4 million years into the future and GL 710 will move within 50000 AU (an Astronomical Unit, or AU, being roughly the distance between the Earth and the Sun). In time’s other direction, the bright Alpha Centauri system of today would not have been visible to the naked eye 3 million years ago.

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Nearest stars in a time range between 20,000 years in the past and 80,000 years in the future.

In this ongoing celestial dance, the closest star will always captivate a technological society looking into life elsewhere and pondering strategies for sending probes across the interstellar gulf. The nearest star is a natural magnet for exoplanet hunters, as is the entire Alpha Centauri system, which comprises Centauri A and B and, if it is indeed gravitationally bound, as seems likely, Proxima itself. What good news that the Pale Red Dot project is now planning a two­-month observing campaign to search for potential Earth-analogs around Proxima Centauri using HARPS, the High Accuracy Radial velocity Planet Searcher spectrograph at the ESO La Silla 3.6m telescope. Nightly monitoring began on January 18th.

Discovered in 1915, by the Scottish astronomer Robert Innes, Proxima Centauri has been kindling imaginations ever since. For science fiction writer Robert Heinlein, it was the inevitable destination of the starship Vanguard, which carried crews that lived and died aboard the ‘generation ship’ in two 1940’s short stories that became his novel Orphans of the Sky. Murray Leinster had earlier claimed the star as our primary target in his 1935 story “Proxima Centauri.” And while Centauri B has recently gotten the lion’s share of attention with the still unconfirmed and now doubtful declaration of a Centauri Bb planetary candidate, Proxima Centauri has had a recent run of study that has helped define the parameters of the planet search.

To Find a Transiting World

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Proxima Centauri.

Some 4.218 light years away from the Sun, this red dwarf star would be obscure even from a planet around Centauri A or B. Separated from them by 15,000 AU, Proxima is small and dim enough that it might take any Alpha Centauri astronomers some time to realize it was close, making the call only once its large proper motion became obvious. A naked eye object, yes, but at magnitude 3.7, it would hardly dominate the sky. Yet it might exert quite an effect on the two larger stars, with Greg Laughlin and Jeremy Wertheimer (UC­Santa Cruz) recently speculating that it could have a role in dislodging comets from the circumbinary disk that presumably surrounds both stars, hence delivering water to their planets.

Whether planets exist around Proxima itself remains an open question. To answer it, various modes of exoplanet detection are being brought into play, the most recent being a transit search by David Kipping’s (CfA) using the Canadian Space Agency’s MOST (Microvariability & Oscillations of STars) space telescope. Begun in the summer of 2014, the project took 13 days of data that year and an additional 30 in 2015. Results are to be announced by the summer of 2016. A small and inexpensive instrument, MOST is best known as the telescope that found transits of 55 Cancri e, making its primary the first naked eye star found with a transiting planet.

A transit detection, tracing the dip in starlight as a planet passed in front of the star as seen from MOST, would put the space telescope in the history books. Transit studies have advantages when it comes to small stars like Proxima Centauri. Proxima’s size is roughly one-tenth that of our Sun. Any habitable planet around it should produce a relatively deep transit signature in the star’s light curve, because the size of the planet in relation to the star is significant as opposed to small worlds around much larger G­- or F-­class stars. For the same reason, the likelihood of a transit alignment is enhanced.

A Planet through Gravity’s Lens

Gravitational microlensing also offers up prospects for tracking down Proxima planets, as noted in 2013 by Kailash Sahu (Space Telescope Science Institute), who realized that a star with such high angular motion across the sky might frequently occult a more distant object. In microlensing, the nearer object creates a ‘lensing’ of the background source as light flows along curved spacetime, an effect predicted by Einstein. An occultation of a distant star by Proxima might allow one or more planets to be revealed as they create their own lensing effect following the occultation by Proxima Centauri itself, slightly brightening the image of the background star.

Sahu found two occultation events, the first being passage in front of a 20th-­magnitude background star in October of 2014, the second an occultation of a 19.5­-magnitude star in February of 2016. Using both, it should be possible to measure Proxima’s mass to an accuracy of five percent. The Hubble Space Telescope, the European Southern Observatory’s Very Large Telescope (Chile) and ESA’s Gaia space telescope are all capable of measuring down to 0.2 milliarcseconds, while the displacement of the two background stars induced by Proxima’s mass is estimated at 0.5 milliarcseconds and 1.5 milliarcseconds respectively.

Probing Stellar ‘Wobbles’

Gravitational microlensing may or may not yield a Proxima Centauri planet, but the star has also been subjected to several radial velocity studies, in which we look for and analyze a characteristic stellar motion. This signal manifests as an extremely faint Doppler shift caused by the effect of an orbiting planet as the star moves slightly further away from us, then closer again. We can track this apparent ‘wobble’ with exquisitely sensitive spectrographs, as Michael Endl (UT­Austin) and Martin Kürster (Max­Planck­Institut für Astronomie) have done for Proxima Centauri using seven years of data from the UVES spectrograph at the Very Large Telescope in Paranal (Chile).

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Upper limit of planetary masses that could have been detected orbiting Proxima Centauri based on observations of M. Endl and M. Kürster. The habitable zone is shown in as a green zone. Image from Endl & Kürster, A&A, 488, 1149.

No planet has been detected, but we’re only part way into the game, for we are beginning to see what kind of planets we can exclude from the realm of possibility. Endl and Kürster find no planet with Neptune’s mass or above, for instance, out to about 1 AU from the star. We can also make a statement about ‘super­-Earths’—rocky worlds more massive than our own—the researchers find no such worlds larger than 8.5 Earth masses in orbits of less than 100 days.

We are not, then, excluding the possibility of planets, but only beginning to declare what we have not yet found. Scientists consider a star’s habitable zone to be the region where liquid water could exist on the surface of a planet. In Proxima Centauri’s case, that zone should reach between 0.022 and 0.054 AU, corresponding to orbits between 3.6 and 13.8 days. The Proxima investigations have yet to find anything in this window, but so far the most we can say is that super­-­­Earths of 2­3 times the mass of the Earth in circular orbits have been ruled out.

With these limits in mind, it’s worth noting an astrometric study, led by G. Fritz Benedict (McDonald Observatory) in the 1990s, used the Hubble telescope to scrutinize the precise position of Proxima Centauri in the sky. In conjunction with a 2013 astrometric study by Lurie (Research Consortium on Nearby Stars), the results produced no planet. These studies indicate that Proxima can have no planet with a mass greater than Jupiter in orbits from 0.14 to 12.6 years.

What Pale Red Dot Might Find

The Pale Red Dot campaign’s radial velocity studies sharpen our focus on a target that is rife with possibilities. What about the prospects for life if we do locate a planet within the Proxima Centauri habitable zone? Here we have two issues to contend with. Like many younger M-­dwarfs, Proxima is prone to sudden, violent flares, producing sudden changes in brightness to Earth observers and cascades of deadly particles for any life forms on a planet. This may or not create an evolutionary niche as creatures adapt themselves over time to the incoming sleet of energetic particles; how such adaptations would succeed can only be speculated about.

Just as significant is the prospect of a planet in the habitable zone being so close to the parent star that it becomes tidally locked, forever putting the same face forward to its star. In a world like this, where the star does not move in the sky, we have permanent night on one presumably very cold side, and permanent day on the other. Fortunately, models developed by Jérémy Leconte (University of Toronto) and colleagues suggest that the presence of an atmosphere can largely overcome this difficulty by distributing hot and cold air so as to moderate temperatures around the planet.

Moreover, 3­D weather simulations by Jun Yang and Dorian Abbot (both of the University of Chicago) and Nicholas Cowan (Northwestern University) show that the side of a tidally locked planet facing the star would develop highly reflective clouds at the ‘sub­stellar’ region directly below the star’s position in the sky. Such cloud coverage could stabilize the atmosphere and produce a cooling effect that bodes well for temperate regions on the day side. There is even the prospect in recent work by Xavier Delfosse (IPAG, Grenoble) that close-­in habitable worlds may be captured into a spin­orbital resonance, but not necessarily into synchronous rotation. The possibility of life on red dwarf planets thus remains open.

Red dwarfs like Proxima Centauri are thought to comprise up to 80 percent of the stars in our galaxy, giving us tens of billions of planets likely to be in the habitable zone of their host stars. Some 100 are relatively close to the Sun, but Proxima retains pride of place as the nearest star to our own. At 4.2 light years, it is a destination we may one day be able to cross using technologies like beamed sails driven by laser or microwaves, but even at a tenth of the speed of light, any probes will take four decades to reach their destination. What could impel us to press ahead is the discovery of a potentially habitable world, a prospect all scientists working on the exoplanet hunt would applaud. The enticing presence of the K-­class Centauri B and solar-like G­-class Centauri A, just 15,000 AU further, is all the more reason we may one day make the crossing.

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Paul Gilster. Photo credit: Paul Gilster.

About the author. Paul Gilster writes and edits Centauri Dreams (http://www.centauri-dreams.org), tracking ongoing developments in interstellar research from propulsion to exoplanet studies and SETI. A full time writer for the last thirty-five years, he is the author of Centauri Dreams: Imagining and Planning for Interstellar Flight (Copernicus, 2004) and Digital Literacy (John Wiley & Sons, 1997). He is also one of the founders of the Tau Zero Foundation and now serves as its lead journalist. This organization grew out of work begun in NASA’s Breakthrough Propulsion Physics program, and now seeks philanthropic funding to support research into advanced propulsion concepts for interstellar missions. Gilster has contributed to numerous technology and business publications, and has published essays, feature stories, reviews and fiction both in and out of the space and technology arena.