The Search for Planets Around Alpha Centauri

One of the earliest television shows I clearly recall watching as a young child was the original run of the scifi classic, Lost in Space. As any aficionado of 1960s science fiction television can tell you, the original destination of the Robinson family in the (then) far off future year of 1997 was a planet orbiting α Centauri. While hardly the first piece of science fiction involving this star system and certainly not the last, it was this show that originally piqued my interest in the real α Centauri and other nearby star systems as a budding astronomer back in the 1970s. And my interest in this system is hardly unique with it being the subject of increasingly intense scrutiny in recent years by astronomers around the globe interested in finding extrasolar planets.

In recent weeks, there seems to have been a flurry of professional papers submitted for publication relating to the search for planets orbiting α Centauri. No doubt part of this is a result of improving detection techniques but it is also likely in response to the discovery of an Earth-size planet orbiting α Centauri B announced in October 2012. In this first post, I start the process of reviewing the recent work relating to the search for planets in the α Centauri system.

Background

The α Centauri system, which is currently the closest known star system to the Sun (and also known by the ancient name, Rigil Kentaurus), contains three stars. At the heart of the system are a pair of Sun-like stars 4.37 light years from us in the southern constellation of Centaurus. They revolve around each other in a moderately eccentric orbit once every 79.9 years with a mean separation of 23.4 AU – roughly similar to the distance between the Sun and Uranus in our solar system. The larger component of this pair, α Centauri A, is a G2V star like the Sun with 1.10 times the mass of the Sun and 1.52 times its luminosity. The smaller component, α Centauri B, is a slightly cooler K2V star with 0.93 times the mass of the Sun and 0.50 times its luminosity. Estimated to be around 6 billion years old, these two stars are among the most Sun-like in our neighborhood.

Diagram roughly illustrating the orbit of α Centauri B around A (compared to the planets in our solar system) and the size of the stars in the α Centauri system compared to the Sun. Click on image to enlarge. (PHL/UPR Arecibo)

About 15,000 AU (or about a quarter of a light year) from this pair of stars is α Centauri C better known as Proxima Centauri because, at a distance of 4.24 light years, it is the closest star to our solar system (and 0.17 light years closer than α Centauri AB). Unlike α Centauri A and B, Proxima Centauri is a very small M5V red dwarf star with an estimated mass of only 0.12 times that of the Sun and 0.0017 times its luminosity. Because of its close proximity to α Centauri AB and its shared apparent motion, it has been generally believed since its discovery in 1915 by Scottish astronomer Robert Innes that Proxima Centauri is bound to the pair of larger, Sun-like stars.

But studies over the past two decades on the relative motions of these stars tends to suggest that Proxima Centauri might not be gravitationally bound to α Centauri AB and instead these stars might be independent members of a stellar moving group that happen to be close to each other at the present time. A study by Jeremy Wertheimer and Gregory Laughlin (University of California Observatories/Lick Observatory) published in 2006 using the best available information on the positions and motions of these stars shows that Proxima Centauri, with a velocity relative to α Centauri AB of 530±140 meters per second, is right on the cusp of being bound to α Centauri AB. The best measurements suggest that Proxima Centauri is near the apastron or most distant point in a highly eccentric orbit around α Centauri AB with an orbital period measured in hundreds of thousands to maybe even millions of years. However, given the measurement uncertainties, Proxima Centauri might be unbound and simply passing by α Centauri AB. Only future improvements in the accuracy of the radial velocity of Proxima Centauri will definitively resolve the issue.

Planets in the α Centauri System

Because of the closeness of the α Centauri system, it is an excellent target for the search for extrasolar planets by a variety of techniques. And given the Sun-like nature of α Centauri A and B, astronomers have been especially interested in identifying potentially Earth-like planets within the habitable zones of these two stars. According to the latest models of planetary habitability by Kopparapu et al., the habitable zone of α Centauri A should extend from 1.17 to 2.06 AU as conservatively defined by the runaway greenhouse and maximum greenhouse limits for an Earth-mass planet, respectively (ignoring the relatively minor effects of α Centauri B). For the somewhat cooler and less luminous α Centauri B, these limits run from 0.69 to 1.24 AU (again ignoring the relatively minor effects of the distant companion’s luminosity).

A number of studies performed over the past couple of decades using a variety of computational techniques have shown that stable orbits exist within these zones despite the presence of a nearby stellar companion whose distance varies from 11.4 to 35.4 AU over its 80-year orbital period (roughly the equivalent of an orbit ranging from Saturn’s to Neptune’s distance from the Sun in our solar system). These conclusions are borne out once again in a new study about the dynamical stability of terrestrial planets orbiting in the habitable zones in the α Centauri system recently submitted for publication by Eduardo Andrade-Ines and Tatiana Michtchenko (University of Sao Paulo, Brazil).

These researchers used a semi-analytical model of a restricted three-body problem to find that planets in Earth-like orbits around either component would be dynamically stable over the lifetimes of either star so long as the inclination of the planets’ orbits relative to that of the stellar components’ orbit is less than 40°. At higher inclinations where planetary orbits tend to become chaotic, stable regions do exist in parameter space but there is a tendency towards higher eccentricity that might complicate planetary habitability. Given the worse case scenario of a completely random orientation of the planetary orbit relative to the stellar components of α Centauri, there is only about one chance in three of the mutual inclination actually exceeding 40°. If there is a tendency for the planes to be more in line with each other, these odds against stable orbits will decrease significantly. For planets much beyond the habitable zones of both stars, the orbits quickly become unstable because of the influence of the other star.

While these results are reassuring to astronomers and habitable planet enthusiasts alike, there is still the unanswered question of whether or not planets can form around α Centauri A and B to begin with. Various studies of planet growth around binary stars in general and the α Centauri system in particular published over the past couple of decades have been fairly evenly split between the likelihood of terrestrial planets forming or not. While it is true that extrasolar planets have already been found orbiting a pair of close binary stars as well as orbiting components of widely separated binary stars, the orbits of α Centauri A and B are in an intermediate distance range where they would be expected to significantly influence the growth of planets in the disk of gas and dust surrounding each other during the planet formation process. While some studies have found that planets should form without any problems within a couple of AU around each star, still others have found that the orbits of planetary embryos are perturbed so much that instead of merging to form Earth-size planets, they tend to collide too violently inhibiting planet growth as a result. Given the current uncertainties in the details of the planet formation process, the only thing to do is to search for planets in the α Centauri system to see if any can be found.

The Case for α Centauri Bb

While a number of attempts to find planets orbiting the stars in the α Centauri system have been made over the years, the first detection was publicly announced on October 16, 2012 by a European team led by Xavier Dumusque. To make their discovery, Dumusque et al. analyzed 459 measurements of the radial velocity of α Centauri B obtained between February 2008 and July 2011 using the HARPS spectrometer on the European Southern Observatory’s 3.6-meter telescope located in La Silla, Chile. Their analysis showed an apparent 0.51 meter per second variation in the radial velocity α Centauri B with a period of 3.24 days. Dumusque et al. interpreted this tiny variation in the radial velocity as being the result of a planet, designated α Centauri Bb, with an orbital radius of 0.04 AU and a minimum mass of 1.1 times that of the Earth (since the inclination of the orbit of planets found by precision radial velocity measurements are not known except by some other means, only the minimum mass can be determined). Because this planet receives over 300 times more energy from α Centauri B than the Earth receives from the Sun, it is impossible for this planet to be habitable in the conventional sense. However, its presence indicates that Earth-size planets can form in the α Centauri system and that others could exist including farther out in the habitable zone.

But as soon as the discovery of α Centauri Bb was announced, professional astronomers began to voice their doubts and emphasized the need for independent confirmation of the discovery. The HARPS team employed a new data reduction technique to tease the small 0.5 m/s radial velocity signature of the planet out of their data set which, in addition to being affected by the 0.8 m/s precision of their instrument, contained natural sources of noise that were on the order of about 1.5 m/s. This natural noise or “jitter” is caused by phenomena on the surface of α Centauri B such as changes in granulation or the star’s convection pattern, starspots, phlages and a host of other surface activity whose signals are modulated by the 38-day rotation period of α Centauri B. The HARPS team carefully filtered out these effects from their data as well as the effects of stray light from the nearby α Centauri A and believe that the likelihood of a false alarm is on the order of 0.1%.

American astronomer Artie Hatzes (Thuringian State Observatory, Germany) performed his own analysis of the publicly available HARPS data set using different data processing techniques to look for the radial velocity signal of α Centauri Bb. He employed a Fourier-based method as well as a local trend filtering technique to take into account the stellar noise. His analysis did find a signal with a period of 3.24 days with an amplitude of 0.42 m/s using either method but the false alarm probabilities were on the order of a few percent. Further analysis of the random noise in the data showed that it had periodicities in the 2.8 to 3.3 day range and amplitudes on the order of half that of the alleged planetary signal. Hatzes concludes in his paper “a better understanding of the noise characteristics in the RV data as well as more measurements with better sampling will be needed to confirm this exoplanet”. Given the recent experience with GJ 581 where subtle stellar magnetic activity was misinterpreted as being the signature of a pair of potentially habitable planets, this seems prudent.

Orbit-Alpha-Centauri-AB-Wikimedia-WHBK_f840

A diagram showing the apparent and actual trajectory of α Centauri B. The near-term minimum separation between α Centauri A and B as seen from the Earth will occur in December 2015. Click on image to enlarge. (Wikimedia Commons / SiriusB)

In addition to HARPS, there are other teams of astronomers performing radial velocity surveys of stars in the southern sky looking for extrasolar planets. A team of astronomers working with the 1.5-meter telescope at Cerro Tolo Inter-American Observatory (CTIO) in Chile are using CHIRON (CTIO Higher Resolution spectrometer) to search for planets orbiting α Centauri A and B with the support of the Planetary Society. Debra Fischer (Yale University) stated that they had insufficient data to detect α Centauri Bb when its discovery was announced in October 2012 and launched an effort to gather much more data at higher rates starting in 2013 aimed specifically at detecting its 3.24-day signal. To date they have not detected α Centauri Bb in their data even though simulations indicate they should have should have at least at a marginal level. Unfortunately, stray light from α Centauri A is decreasing the quality of their data as the apparent separation of α Centauri A and B closes to a near-term minimum of 4 arc sec in December 2015. Data collected by HARPS and other teams are all similarly affected and it has been generally assumed that astronomers will need to wait a few years for the stars’ separation to increase before confirming α Centauri Bb.

But things might not be that bleak after all. A paper was recently submitted for publication by Christoph Bergmann (University of Canterbury, New Zealand) and a team of astronomers performing a survey using the HERCULES spectrometer on the one-meter McLellan Telescope at the Mt. John University Observatory in New Zealand. The paper describes a new method this New Zealand-based team have developed to significantly reduce the effects of stray light in precision radial velocity measurements. They have found that they can accurately determine the amount of contamination in each of their observations by measuring the relative strength of the hydrogen-α and sodium-D lines of the stellar spectra. They have also modified their analysis software to simultaneously fit two spectral “templates” – one for the star and another for the “contamination” – to model their observations and extract accurate radial velocity measurements. They tested their technique on four double-line spectroscopic binaries (i.e. pairs of unresolved stars that can only be differentiated by periodic shifts in their Doppler-shifted spectral lines) whose blended images represents the worse-case scenario of “contamination”. Bergmann et al. successfully recovered the radial velocities of both components of the binaries they observed and plan to use this new technique in their continuing survey of α Centauri in hopes of confirming α Centauri Bb. Perhaps the HARPS, CHIRON and other teams will employ this or a similar technique to confirm the existence of α Centauri Bb sooner rather than later as well.

The next essay in this series, “”The Search for Planets Around Alpha Centauri II“”, examines the results of searches for planets orbiting α Centauri A and B and plans for the near future. Search results for Proxima Centauri are summarized in “The Search for Planets Around Proxima Centauri“.

General References

Eduardo Andrade-Ines and Tatiana A. Michtchenko, “Dynamical Stability of Terrestrial Planets in the Binary α Centauri System”, arXiv: 1408.1053 (accepted for publication by Monthly Notices of the Royal Astronomical Society), August 5, 2014 [Preprint]

Christoph Bergmann et al., “Searching for Earth-mass planets around α Centauri: precise radial velocities from contaminated spectra”, arXiv: 1407.6415 (accepted for publication in the International Journal of Astrobiology), July 24, 2014 [Preprint]

Bruce Betts, “Update on the search for planets in the Alpha Centauri system”, The Planetary Society Blogs, April 4, 2014 [Link]

Camille M. Carlisle, “Planet Found in Alpha Centauri System”, Sky & Telescope web site, October 17, 2012 [Link]

Xavier Dumusque et al., “An Earth-mass planet orbiting α Centauri B”, Nature, Vol. 491, pp. 207-211, November 8, 2012

Artie P. Hatzes, “Radial Velocity Detection of Earth-Mass Planets in the Presence of Activity Noise: The Case of α Centauri Bb”, The Astrophysical Journal, Vol. 770, No. 2, Article ID 133, June 2013

R. K. Kopparapu et al., “Habitable zones around main-sequence stars: new estimates”, The Astrophysical Journal, Vol. 765, No. 2, Article ID 131, March 10, 2013

Ravi Kumar Kopparapu et al., “Habitable zones around main-sequence stars: dependence on planetary mass”, Astrophysical Journal Letters, Vol. 787, No. 2, Article ID L29, June 1, 2014

Jeremy G. Wertheimer and Gregory Laughlin, “Are Proxima and α Centauri Gravitationally Bound?”, The Astronomical Journal, Vol. 132, No. 5, pp. 1995-1997, November 2006