Alpha Centauri Bb Five Years Later – The Search for Exoplanets Continues

The nearby α Centauri system is among the best known among astronomers and the general public alike. Frequently the setting of various science fiction stories over the decades, our neighbor has been the subject of keen scientific interest because of the Sun-like nature of the system’s largest members and their comparative proximity to us. Since it is also the nearest star system to our solar system, α Centauri is also a logical target for studies of future interstellar travel including, most recently, the Breakthrough Initiative which proposes to send an ultra-light, laser propelled spacecraft on a half century long mission.

Interest in α Centauri only increased on October 16, 2012 when a European team of astronomers led by Xavier Dumusque (Observatoire de Genève) announced the discovery of an exoplanet designated α Centauri Bb. The discovery team calculated that this new find was an Earth-size world but its tight orbit around its sun would render it a super-hot version of Venus. While not the Earth-like world many had hoped to find, its discovery offered the prospects that such worlds could exist and might soon be detected as well. Unfortunately, events in the years following the discovery of α Centauri Bb offers us yet another example of just how difficult finding such exoplanets still is even after decades of improvements in astronomical instrumentation and techniques.

Background

The α Centauri system is the closest known to the Sun and consists of three stars. On November 6, 2016 the International Astronomical Union (IAU) officially adopted the ancient name for this star, Rigel Kentaurus – a Latinization of the Arabic name رجل القنطورس or Rijl al-Qanṭūris meaning “foot of the centaur”. At the heart of this system are a pair of Sun-like stars 4.37 light years from us that are locked in a moderately eccentric, 80-year orbit around each other. The larger component of this pair, α Centauri A, is a G2V star like the Sun with 1.10 times the Sun’s mass 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 5 billion years old, these two stars are among the most Sun-like in our neighborhood.

This diagram shows the relative sizes of the Sun compared to the known members of the α Centauri system. Click on image to enlarge. (PHL)

About 13,000 AU (or about a quarter of a light year) from this pair of stars is α Centauri C better known by its now official IAU name (as of August 21, 2016) of Proxima Centauri because, at a distance of 4.24 light years, it is the closest star to our solar system. Unlike α Centauri A and B, Proxima Centauri is a very small M5.5V red dwarf star with an estimated mass of only 0.12 times that of the Sun and 0.00156 times its luminosity. On August 24, 2016, an international team of astronomers led by Guillem Anglada-Escudé (Queen Mary University of London) as part of the Pale Red Dot campaign announced the discovery of Proxima Centauri b – an Earth-size exoplanet orbiting inside of the habitable zone of this nearby red dwarf (see “Habitable Planet Reality Check: Proxima Centauri b”). Based on the precision radial velocity (RV) measurements used to make this discovery, the presence of a second exoplanet was suspected and a new campaign called Red Dots made follow up observations during the summer of 2017 to help settle the question of its existence (see “Proxima Centauri b One Year Later: The Search for More Exoplanets Continues”).

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 α Centauri AB also appears in the image to the upper-right of Proxima itself. (ESO/M. Kornmesser)

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 orbits the pair of larger, Sun-like stars. It was not until recently that Pierre Kervella (Unidad Mixta Internacional Franco-Chilena de Astronomía/Observatoire de Paris), Frederic Thévenin (Observatoire de la Côte d’Azur) and Christophe Lovis (Observatoire Astronomique de l’Université de Genève) were able to prove definitively that all three stars are gravitationally bound. Using the latest data on the positions and motions of the stars as well as corrections for how precision measurements of absolute RV are affected by various stellar phenomena, Kervella et al. were able to show that Proxima Centauri is locked in an eccentric orbit which ranges from 4,300 to 13,000 AU with a period of about 550,000 years (see “The Orbit of Proxima Centauri”).

A 3D diagram showing the orbit of Proxima Centauri around α Centauri AB as derived by Kervella et al.. Click on image to enlarge (Kervella et al.)

Despite the distance between α Centauri A and B varying from 11 to 35 AU during the course of one revolution, various dynamical studies performed over the decades have predicted that regions with stable planetary orbits do exist in this system. These studies have shown that orbits around α Centauri A or B out to as far as about 3 AU would be stable depending on their inclination with respect to the plane of the stars’ orbit. What has not been so clear is if planets could form around this pair of stars.

A number of studies performed over the past couple of decades have been more or less evenly split on the question of whether or not planets could form around α Centauri A and B and other binary systems like it. Some studies have shown that the building blocks for planets, called planetesimals, would be able to collect themselves together into planets out to some reasonable distance. Still other studies have suggested that the presence of the two stars would have stirred up the orbits of the planetesimals too much. Instead of collecting into larger bodies, the planetesimals would tend to smash themselves apart upon contact so that planets could not form. In the end, the best way of resolving this question was to look for planets.

The Discovery α Centauri Bb

Given the technological challenges of detecting exoplanets even in a nearby star system like α Centauri, the first technology that offered a reasonable chance of success was the precision measurement of changes in the stars’ radial velocity (RV) resulting from the small reflex motion of an orbiting exoplanet. But after almost two decades of measurements with increasingly accurate instruments and refined techniques, the results of searches for exoplanets orbiting α Centauri A and B published up to the opening decade of this century had found nothing. This null result combined with dynamical arguments only demonstrated that planets larger than Saturn or Jupiter did not orbit within about 2 AU of either α Centauri A or B. This still left a lot of possibilities including Earth-size exoplanets orbiting comfortably inside the habitable zones of these stars but much more precise radial velocity measurements would be required to detect them (for a more detailed account of these earlier RV searches performed up to this point, see “The Search For Planets Around Alpha Centauri – II”).

Beginning about a decade ago, several teams employing various observing approaches are known to have started looking for lower-mass exoplanets orbiting α Centauri A and B with instruments capable of making RV measurements with uncertainties as good as on the order of one meter per second – a factor of up to four more precise than in any previously published results for the system. The first team to announce the results from their search was the European team using the HARPS (High Accuracy Radial Velocity Planetary Searcher) spectrometer on the 3.6-meter telescope at the European Southern Observatory in La Silla, Chile. A new data processing technique was employed on the 459 RV measurements they obtained of α Centauri B between February 2008 and July 2011. These RV data had a measurement uncertainty of 0.8 meters per second and contained an estimated 1.5 meters per second of natural noise or “jitter” resulting from a range of activity on the surface of α Centauri B.

The ESO 3.6m Telescope equipped with HARPS used to make precision RV measurements to search for exoplanets. (ESO/H.H.Heyer)

On October 16, 2012 Dumusque et al. announced the discovery of an exoplanet designated α Centauri Bb based on periodic variation in the RV with a semiamplitude of just 51 centimeters per second revealed by their analysis. Their results suggested an exoplanet with a minimum mass or M_psini (where i is the unknown inclination of the planet’s orbit to the plane of the sky) of just 1.1 times that of Earth locked in a tight orbit with a period of 3.24 days and a radius of 0.04 AU. This was well below the upper limits set by earlier searches. Given sufficient observation time, Dumusque et al. estimated in their discovery paper that in the future they could detect an exoplanet with a M_psini of about four times that of the Earth in a 200-day orbit inside the habitable zone of α Centauri B (for a more detailed account of the discovery of α Centauri Bb, see “The Search For Planets Around Alpha Centauri”).

A plot of RV as a function of orbital phase of α Centauri Bb. The gray dots represent individual data points while the red symbols denote binned and averaged RV values with their estimated uncertainty. The curve shows the best fit to the data. Click on image to enlarge. (Dumusque et al.)

While the HARPS team’s result generated much excitement, it was also met with a healthy amount of skepticism in the astronomical community because of the new mathematical technique used to process the data to extract such a low-amplitude RV signal. One of the first critics, American astronomer Artie Hatzes (Thuringian State Observatory, Germany), performed his own analysis of the publicly available HARPS data set using two more widely employed data processing methods to look for the RV signal of α Centauri Bb. Formally published in June 2013 in The Astrophysical Journal, Dr. Hatzes’ analysis did indeed find a signal buried in the RV data with a period of 3.24 days but it had a false alarm probability of a few percent – far too high to be considered a reliable detection. Furthermore, his 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.

Given the situation of potential planetary false alarms with GJ 581, GJ 667C and Kapteyn’s Star, this finding suggested that noise in the data irregularly sampled over time, especially from activity on α Centauri B, might have been mistaken for a planet (see “Habitable Planet Reality Check: GJ 581”, “Habitable Planet Reality Check: GJ 667C” and “Kapteyn b: Has Another Habitable Planet ‘Disappeared’?”). Dr. Hatzes concluded that additional data were needed to better understand the nature of the noise in the RV measurements and confirm the planetary nature of the RV signal. Despite the criticism, Dumusque et al. continued to stand by their results and the mathematical rigor of their new technique used to find α Centauri Bb.

The Loss of α Centauri Bb

Other teams made RV measurement in order to confirm the existence of α Centauri Bb as part of their ongoing observing programs. A team of astronomers working with the 1.5-meter telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile were using CHIRON (CTIO Higher Resolution Spectrometer) to search for planets orbiting α Centauri A and B in part with the support of The Planetary Society. The project’s principal investigator, Debra Fischer (Yale University), was quoted in a blog on The Planetary Society’s web site posted in April 2014 by Bruce Betts that her team had insufficient data to detect α Centauri Bb when its discovery was announced in October 2012. They soon launched a renewed effort to gather much more data at a higher cadence starting in 2013 aimed specifically at detecting the purported planet’s 3.24-day signal. They did not detect α Centauri Bb in their data but their simulations at the time indicated that any such detection would have been marginal at best.

One of the issues complicating continuing efforts to gather more the precision RV measurements needed to resolve the situation with α Centauri Bb was the amount of stray light from α Centauri A that degrades the quality of  RV measurements. As viewed from the Earth, the apparent separation of α Centauri A and B had been decreasing at an accelerating rate for about a third of a century as the stars move in their inclined elliptical paths around each other. The two reached their near-term minimum apparent separation of only four arc seconds in December 2015 and finally started moving apart afterwards easing the issue of getting more data.

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 occurred in December 2015. Click on image to enlarge. (Wikimedia Commons/SiriusB)

But before new data could be acquired and analyzed, the validity of the α Centauri Bb discovery claim had already been refuted. In a preprint posted on October 19, 2015 (just three days after the third anniversary of the α Centauri Bb discovery announcement), Vinesh Rajpaul, Suzanne Aigrain and Stephen J. Roberts (University of Oxford) presented a new analysis of the RV data for α Centauri B. Rajpaul et al. found that the irregular spacing over time of the RV data set resulted in a window function that created a spurious signal or ghost which mimicked the signature of an orbiting exoplanet with a period of 3.24 days. Even in synthetic data with a time sampling identical to the original but with no RV variation close to the apparent period of α Centauri Bb present, Rajpaul et al. found that the ghost signal continued to appear. There was now no doubt that α Centauri Bb (as described by Dumusque et al.) did not exist and even Xavier Dumusque publicly conceded this unfortunate fact.

With the existence of α Centauri Bb as described by Dumusque et al. disproved, it was back to square one for the HARPS team and others searching for exoplanets orbiting these Sun-like stars. Additional data, improved observing strategies and better data processing techniques still promise to yield detections of RV signals consistent with super-Earth to Neptune size exoplanets orbiting either α Centauri A or B especially as the apparent separation of these stars continues to increase over the next dozen years. Also in the works are a new generation of improved instruments which should push the RV measurement uncertainties down to 10 centimeters per second and even better making even the detection of Earth-size exoplanets in the habitable zone theoretically possible.

But even with these new and more precise RV measurements, the definitive identification of the signature of an orbiting exoplanet might not be so easy. Natural jitter as well as other sources of noise in the RV measurements and how it is taken into account in the mathematical analysis of the data has complicated matters. For example, one new processing technique was recently developed by a collaboration of astronomers involved in a number of the world’s exoplanet search programs and was applied to some publicly available RV data sets. In a paper published in October 2017 issue of The Astrophysical Journal with Fabo Feng (University of Hertfordshire, UK) as the lead author, the group described how they found evidence for four exoplanets orbiting the Sun-like star τ Ceti including two which appear to straddle the habitable zone (HZ) of this Sun-like star (see “Habitable Planet Reality Check: Tau Ceti”). Despite the apparent robustness of the detection of periodic variations in the RV with semiamplitudes of 35 to 55 centimeters per second, Feng et al. are still cautious about the planetary interpretation of these signals. In this new realm, the data could be revealing previously unrecognized types of stellar activity and other sources of spurious signals can not be excluded. Even if a periodic variation in the RV is detected, an independent means of verifying the planetary nature of the find will surely be required.

The Transit Method

One of the other methods currently available to detect exoplanets is the transit method. This method relies on precision photometric measurements to reveal the periodic dips in a star’s apparent brightness as a result of an orbiting exoplanet passing almost directly between us and the star blocking some of the host’s light in the process. This method has been used successfully by NASA’s Kepler mission to detect thousands of exoplanets (see this site’s Kepler page for more background). Unfortunately, the probability that any given exoplanet has its orbit aligned by random chance to produce observable transits is low and decreases inversely with the size of the semimajor axis of the exoplanet. For an exoplanet in an Earth-like orbit around a Sun-like star, the odds that the orbit would be aligned to produce observable transits is only about 0.5%. This is why Kepler has had to observe hundreds of thousands of stars during its primary and now extended “K2” mission to detect the number of exoplanets it has. While the odds of detection are not good, the transit method does offer the opportunity to independently verify the existence of an exoplanet detected using precision RV measurements using available technology.

A view of the Hubble Space Telescope in orbit above the Earth. (STSci/NASA)

In 2013 and 2014, a group of astronomers employed the Hubble Space Telescope (HST) to search for transits caused by α Centauri Bb in order to verify its existence. Because of the small orbital radius of this purported exoplanet compared to the size of the star it orbits, there was a 9.5% probability that its orbit would be oriented by random chance to produce transits as viewed from the Earth. Assuming an Earth-like density for α Centauri Bb, such transits would be expected to reduce the apparent brightness of α Centauri B by about 100 parts per million (ppm). While obtaining this level of photometric accuracy is not possible using any ground-based instruments, orbiting telescopes like HST are capable of making the required observations.

In June 2015, a paper by an international collaboration of scientists (including five of the 11 authors of the original α Centauri Bb discovery paper) with Brice-Olivier Demory (Cavendish Laboratory) as the lead author was published in The Monthly Notices of the Royal Astronomical Society which presented an analysis of photometric measurements of α Centauri B made using the HST. To make their measurements of α Centauri B, Demory et al. used the Space Telescope Imaging Spectrograph (STIS) which was installed on HST during its second servicing mission in 1997 and was subsequently repaired in 2009.

STIS made almost continuous observations of α Centauri B for 26 hours from July 7 to 8, 2013 around the time a transit of α Centauri Bb was expected. Detailed analysis of the photometric measurements corrected for various instrumental effects yielded an accuracy of about 115 ppm for individual brightness measurements with greater accuracy possible by analyzing the thousands of data points collectively. After a full analysis, a very promising transit-like event about 3.8 hours long with a depth of about 90 ppm was detected in the data consistent with the transit of a planet with 0.92±0.06 times the radius of the Earth.

Plots showing the transit event observed using HST’s STIS in July 2013 (top panel) and the residuals after the data are fitted to a transit model (bottom panel). The gray dots are individual data points while the back dots are data binned into 45-minute increments. Click on image to enlarge. (Demory et al.)

With this apparently positive result, the team was able to schedule another 13.5 hours of uninterrupted observation time on HST between July 28 to 29, 2014 to reobserve α Centauri B in hopes of spotting another transit event. Employing the same data reduction and analysis procedures used for the 2013 HST data, Demory et al. observed no transit-like events in the newer data set. Given the quality of the data, a 3.8-hour long transit with a depth of about 100 ppm like that seen in 2013 should have been detected to 5σ level or better but none was present. It now seemed unlikely that a transit of α Centauri Bb had been observed in 2013 after all. But given the low probability that α Centauri Bb would produce an observable transit, this null result alone could not be taken as proof this exoplanet did not exist.

A detailed analysis of the 40 hours of available HST photometric measurements from 2013 and 2014 has ruled out all explanations for the observations save for one: α Centauri B is orbited by a different Earth-size exoplanet that is not the now-refuted α Centauri Bb. The best fit for the limited data hints that this new exoplanet candidate is in a tight orbit around its primary with an orbital period probably no greater than about 20 days and likely to be around 12 days (for more details on these observations and analysis, see “Has Another Planet Been Found Orbiting Alpha Centauri B?”).

An artist’s depiction of NASA’s TESS satellite which will systematically search the bright stars of the sky for planetary transits starting in 2018. (NASA)

Unfortunately with such a poorly constrained orbit, three weeks of nearly continuous photometric monitoring of α Centauri B will be required to confirm this hypothesis. HST is too busy to accommodate a dedicated search of this length and no other space telescope currently available is capable of making the needed observations especially with the apparent separation of α Centauri A and B being so small. In addition, since the RV signature for this planet would be expected to be maybe a couple of tens of centimeters per second, this method has little likelihood of providing independent confirmation of this sighting any time soon. We will have to wait for new space-based telescopes to become available such as NASA’s TESS (Transiting Exoplanet Survey Satellite) mission or ESA’s CHEOPS (Characterizing Exoplanets Satellite) which are both scheduled for launch in 2018 and may be capable of making the required observations of such a bright target.

Direct Imaging

There is yet another method that is becoming available to detect exoplanets orbiting the Sun-like stars of the α Centauri: direct imaging. As is the case with other exoplanet search techniques, the proximity of α Centauri makes it an ideal candidate for direct imaging of extrasolar planets. Compared with other nearby Sun-like stars, exoplanets orbiting either α Centauri A or B would be brighter and have a larger apparent separation from their bright host stars, all else being equal.

One of the most sensitive direct-imaging searches for small companions in the α Centauri system was performed by Kervella and Thévenin. They used the SUSI2 (Super Seeing Imager 2) camera on ESO’s 3.6-meter NTT (New Technology Telescope) to obtain sequences of images in 2004 and 2006 in the V, R, I and Z bands (corresponding to wavelengths of 551, 658, 806 and 900 nm, respectively). They were looking for dim objects a few tens of arc seconds or more away from and out of the glare of α Centauri A and B.  Such a distant object would not orbit either star alone in what is called an “S-type” orbit.  Instead it would be orbiting both stars around the barycenter of the α Centauri AB system in what is called a “P-type” orbit much as Proxima Centauri is now known for certain to do.

A false color image made from the VRI-filtered SUSI2 images of the region around α Centauri AB acquired in 2004. The glow in the center of the image is the point spread function of the bright stars with the scattered dots being dim background stars. The scale bar in the lower left is one arc minute. (Kervella & Therenin)

Kervella and Thévenin found no faint comoving companions of α Centauri AB in their search eliminating the possibility of objects with a mass greater than 15 M_J orbiting α Centauri AB at distances of 100 to 300 AU (corresponding to orbital periods of about 1,400 to 7,400 years) as well as objects larger than 30 M_J between 50 and 100 AU (with orbital periods of 500 to 1,400 years). Earlier dynamical studies have shown that objects much closer to α Centauri AB would not have a stable orbit. The presence of Proxima Centauri with a periapsis recently pegged at about 4,300 AU would likely affect the long term stability of any bodies with orbits much larger than this. This search thus precludes the existence of small to moderate-mass brown dwarfs in comparatively close-in P-type orbits around α Centauri AB.

In order to image planets closer to α Centauri A and B, more advanced imaging methods are required. In a conference paper present in 2014 with Jared Males (Steward Observatory) as the lead author, a study on the ability to use adaptive optics (AO) on Earth-based telescopes to image planets in the habitable zones of nearby stars was presented. And to illustrate the potential power of modern AO techniques, they used actual images of α Centauri A obtained in April 2014 as a test case. For their exercise, they used the 6.5-meter Magellan AO telescope equipped with the VisAO camera operating at a wavelength range of 0.4 to 1.0 μm and the Clio2 infrared camera operating at 1 to 5 μm. Males et al. used the images they obtained of α Centauri A to perform simulations that demonstrated that they could detect exoplanets with contrast ratios (i.e. the ratio between the brightness of the exoplanet and the star it orbits) in the 10^-6 to 10^-7 range inside the habitable zone of α Centauri A which ranges from about 0.55 to 1.55 arc seconds from the star as seen from Earth. They believe that with improvements to their AO system and imaging techniques, they should be able to detect exoplanets with contrast ratios of as low as about 10^-8.

This plot shows the apparent brightness of a hypothetical Earth-like planet in the habitable zone (denoted by the relative size of the symbol) of various nearby stars as a function of apparent separation and the contrast ratio. Also included in the upper right are a sample of exoplanets which have already been directly imaged. The types of instruments required for various ranges of separation and contrast ratio are also indicated. Click on image to enlarge. (Belikov et al.)

Unfortunately, an Earth-like exoplanet in the habitable zone of a Sun-like star such as α Centauri A would have a contrast ratio on the order of about 10^-10 which is far beyond what any ground-based system could achieve. In fact, even a Jupiter-like exoplanet would not be detectable orbiting near the habitable zone of α Centauri A with a system limited to a contrast ratio in the 10^-6 to 10^-7 range. However, with the hoped-for detection threshold of 10^-8, even a Neptune-size planet would be expected to be detectable near the habitable zone of α Centauri A. This and similar ground based adaptive optics imaging systems could soon search for Neptune-size worlds in this system.

The detection of still smaller exoplanets orbiting α Centauri A or B (including unconfirmed detections found using RV measurements) will require the development of a space-based system. Fortunately, such a telescope could be of relatively modest size due to the comparatively high apparent brightness and large separation of habitable zone exoplanets in the α Centauri system as viewed from the Earth. A couple of years ago, Edward Bendak and Ruslan Belikov (NASA Ames Research Center) along with Males and others teamed up to propose the small Explorer-class ACEsat (Alpha Centauri Exoplanet Satellite) mission. With a modest price tag pegged at about $175 million, this satellite would have employed a specially designed 45-centimeter space telescope fitted with a coronagraph to spot Earth-size exoplanets orbiting in the habitable zones of α Centauri A and B.

This is a sequence of simulated ACEsat images over time showing our solar system’s inner planets if they orbited α Centauri A. The planets Venus (V), Earth (E) and Mars (M) are clearly visible especially when the images are stacked as shown in the three bottom row images. Click on image to enlarge. (Belikov et al.)

Placed in a solar orbit trailing the Earth, ACEsat could have observed α Centauri A and B for two years following a planned 2020 launch. While ideal for spotting Earth-size exoplanets in the habitable zone, larger space-based telescopes will need to be developed to spot the possible exoplanet which was observed transiting α Centauri B in 2013 (not to mention habitable zone planets of more distant and dimmer stars). Potentially a space-based telescope like JPL’s proposed Habitable Exoplanet Imaging Mission (HabEx) may be able to spot such closely orbiting exoplanets.

Despite the modest cost compared to other space telescope projects, NASA did not select the ACEsat proposal. After this setback, members of the ACEsat team and other supporters continued to push for the development of this type of instrument. Out of the ashes of ACEsat came Project Blue – a privately funded initiative out of the Boldly Go Institute in partnership with the SETI Institute, Mission Centaur, and UMass Lowell (this author’s alma mater, coincidentally). In order to raise the $175,000 to start the project, Project Blue has set up an Indiegogo campaign which can be found here. The hope is that their $50 million space telescope can be launched in 2021. With luck, Project Blue or similar instruments could start the direct imaging search for Earth-size exoplanets orbiting inside the habitable zones of α Centauri A and B sometime in the first half of 2020. Combining data from these finds with those from RV searches and other programs could give us our first real look at potentially habitable Earth-size exoplanets that might reside in the α Centauri system.

An artist’s concept of the space telescope being proposed by Project Blue to search for Earth-size exoplanets orbiting α Centauri A and B. (Project Blue)

Summary

Although the existence of α Centauri Bb found by Dumusque et al. a half decade ago has been disproved by further analysis, the future prospects for finding exoplanets orbiting α Centauri A and B are good and continue to improve. With the apparent separation of α Centauri A and B increasing over the next dozen years, it should be once again possible to gather new precision RV measurements in search of exoplanets. Very soon it should be possible to push the detection threshold well below the current published limit of Jupiter to Saturn-size objects. With constantly improving observing strategies, data processing techniques and the introduction of a new generation of instruments capable of even more precise RV measurements, the detection of Earth-size exoplanets orbiting in the habitable zone will be theoretically possible. Unfortunately, any such detection will likely require confirmation by independent methods to ensure that the observed RV signature is planetary in nature and not the result of noise or some subtle artifact in the data.

Fortunately such independent techniques are also becoming available. The transit method employed so successfully by NASA’s Kepler mission could be used to detect Earth-size exoplanets around α Centauri A or B assuming that their orbits are align by chance to be nearly edge on as viewed from the Earth. The  detection of what appears to be a transit-like event in 2013 by Demory et al. using the Hubble Space Telescope suggests that maybe one such transiting Earth-size exoplanet exists orbiting α Centauri B. NASA’s TESS and ESA’s CHEOPS missions to survey all the bright stars of the sky looking for exoplanet transits promise to provide the data to confirm the presence of this exoplanet as well as search for any other transiting worlds that may be present.

By far the most promising prospects come from direct imaging techniques. The performance of ground-based telescopes fitted with state-of-the-art adaptive optics systems should soon improve to the point where Jupiter to Neptune-size exoplanets could be detected in the habitable zones of α Centauri A and B. The comparatively modest Project Blue, which proposes to launch a 45-cm telescope purpose-built to observe α Centauri, could be launched as soon as 2021 to begin searching for Earth-size exoplanets. More capable space-based instruments of the future also promise to add much more to our knowledge of these worlds if they are found. Within the next few years, we may finally find confirmed exoplanets orbiting the Sun-like stars of the of α Centauri system and targets for future exploration.

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Related Reading

For more articles on this system, check out Drew Ex Machina’s α Centauri Page.

General References

R. Belikov et al., “How to directly image a habitable planet around Alpha Centauri with a ~30-45 cm space telescope”, Proceedings of the SPIE, Vol. 9605, ID 960517, 2015

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

Brice-Olivier Demory et al., “Hubble Space Telescope search for the transit of the Earth-mass exoplanet Alpha Centauri Bb”, Monthly Notices of the Royal Astronomical Society, Vol. 450, No. 2, pp. 2043-2051, June 2015

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

M. Endl et al., “The planet search program at the ESO Coude Echelle spectrometer II. The α Centauri system: Limits for planetary companions”, Astronomy and Astrophysics, Vol. 374, pp. 675-681, August 2001

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

P. Kervella and F. Thevenin, “Deep imaging survey of the environment of α Centauri. II. CCD imaging with the NTT-SUSI2 camera”, Astronomy and Astrophysics, Vol. 464, No. 1, pp.373-375, March II 2007

P. Kervella, F. Thévenin and C. Lovis, “Proxima’s orbit around Alpha Centauri”,  Astronomy & Astrophysics, Vol. 598, ID L7, February 2017

Jared R. Males et al., “Direct imaging of exoplanets in the habitable zone with adaptive optics”, Proceedings of the SPIE, Vol. 9148, ID 914820, 2014

V. Rajpaul et al., “Ghost in the time series: no planet for Alpha Cen B”, The Monthly Notices of the Royal Astronomical Society – Letters, Vol. 456, No. 1, pp L6-L10, February 2016