Has Another Planet Been Found Orbiting Alpha Centauri B?

About two and a half years ago, the web was filled with stories about the announcement of an Earth-size extrasolar planet discovered orbiting our Sun-like next door neighbor, α Centauri B (see “Happy Anniversary Alpha Centauri Bb?”). But as the initial buzz about the planet α Centauri Bb began to fade, it was realized that this discovery was not on as firm a footing as many earlier extrasolar planetary discoveries and that independent confirmation of this find was not likely to be available anytime soon. New observations using a different method of planet detection have recently become available but, instead of providing evidence that could confirm the presence of α Centauri Bb, they seem to have only added to the questions surrounding our neighboring star 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 its heart are a pair of Sun-like stars 4.37 light years from us that are locked in an 80-year orbit around each other. The larger component of this pair, α Centauri A, is a G2V star 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. The third member of this system is known as Proxima Centauri since it is slightly closer to us at a distance of 4.24 light years. This more distantly orbiting component of the system 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 (see “The Search For Planets Around Proxima Centauri”).

The discovery of the extrasolar planet α Centauri Bb was publicly announced on October 16, 2012 by a Swiss-based European team led by Xavier Dumusque of the Geneva Observatory. To make their discovery, Dumusque et al. had 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 apparently regular 0.51 meter per second variation in the radial velocity of α Centauri B with a period of just 3.24 days. Dumusque et al. interpreted this tiny variation as being the result of a planet with an orbital radius of 0.04 AU and a minimum mass of 1.1 times that of the Earth. Since the inclination of this planet’s orbit to the plane of the sky can not be determined by radial velocity measurements alone, its actual mass is currently unknown but is probably larger than its derived minimum mass. A recent paper submitted for publication by Plavchan et al. makes dynamical arguments that suggest that the actual mass of α Centauri Bb is probably no greater than about 2.7 times that of the Earth, assuming their formation scenario is correct.

Given the small amplitude of the radial velocity variation associated with α Centauri Bb compared to the much greater natural noise or “jitter” in the radial velocity of α Centauri B and the novel nature of the data processing technique used by Dumusque et al. to extract that signal, there has been some healthy skepticism in the astronomical community about the planetary interpretation of their data. Independent observations and analysis are required to verify this important discovery. But because of the already small and quickly decreasing separation between α Centauri A and B which has made follow up measurements increasingly difficult, α Centauri Bb has yet to be independently confirmed by other groups using radial velocity measurements (for a detailed discussion about the discovery of α Centauri Bb and the attempts being made to confirm it, see “The Search For Planets Around Alpha Centauri”).

New Observations

As we wait for the results from new radial velocity measurements, there are other techniques currently available that are capable of independently verifying the existence of α Centauri Bb. Because of the small orbital radius of α Centauri Bb compared to the size of the star it orbits, there is a 9.5% probability that its orbit is oriented by random chance to produce observable transits of the planet across the face of α Centauri B 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 at least about 100 parts per million (ppm). While obtaining this level of photometric accuracy is not possible using any ground-based instruments, there are orbiting telescopes that are capable of making the required observations.

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

Recently, 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 of the Cavendish Laboratory as the lead author has been accepted for publication which presents an analysis of photometric measurements of α Centauri B made using the Hubble Space Telescope (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 measurements of α Centauri B for 26 hours from July 7 to 8, 2013. Great pains were taken to minimize the amount of light contamination from α Centauri A which was only 4.5 arc seconds away at the time. A total of 2,087 six-second exposures were made using STIS which allowed the brightness of α Centauri B to be monitored for about 96% of the transit window predicted using the orbital solution calculated by Dumusque et al. 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.

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 after all.

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.)

A more thorough analysis of the statistically significant transit-like signature observed in July 2013 demonstrated that it was not likely caused by a transit of α Centauri Bb. By combining just the 2013 photometric data with the earlier radial velocity results, Demory et al. found that the orbit of α Centauri Bb would need to have an eccentricity of 0.54 to explain the length of the observed event. However, given the small orbital radius of α Centauri Bb, tidal damping should have circularized its orbit in less than 100 million years – significantly less that the estimated 5 to 6 billion year age of the α Centauri system. Combined with the lack of a transit event in July 2014, it is improbable that α Centauri Bb was responsible for what was observed in 2013. In fact, the transiting nature of α Centauri Bb has been ruled out to a 96.6% confidence level, assuming it has the orbital parameters as determined by Dumusque et al..

Additional investigation of the transit-like event of July 2013 effectively rules out the possibility that it was caused by STIS instrumental effects. Likewise, the signature of the event is not consistent with starspots or any other forms of stellar activity on α Centauri B itself. There are also no signs of contamination in the data by light from α Centauri A. Based on the 40 hours of available data, it seems that the event observed in July 2013 was not caused by α Centauri Bb but by another Earth-size planet orbiting α Centauri B with an orbital eccentricity less than 0.24 and a period of no greater than 20.4 days to 2σ certainty (which corresponds to an orbital radius of no greater than 0.14 AU). The median value for the orbital period derived by the analysis by Demory et al. is 12.4 days yielding an orbital radius of about 0.10 AU – far too close to its sun to be habitable in any conventional sense.

Despite the convincing nature of this single observation, Demory et al. are not formally claiming to have discovered another planet but they do call for additional observations. Using independent methods like precision radial velocity measurements to confirm this potential find are unlikely to yield useful results anytime soon. The radial velocity signature of an Earth-size planet in an orbit with a period on the order of 10 to 20 days long would be even smaller than that of the still unconfirmed α Centauri Bb. More accurate radial velocity data will be required to do that (see “The Search For Planets Around Alpha Centauri – II ”).

Unfortunately, the near term prospects of reobserving this potential “α Centauri Bc” photometrically are not very good either. With its orbital period so poorly constrained, about three weeks of nearly-continuous photometric observations with an accuracy of about 100 ppm will be required to observe any transit assuming the planet actually exists. As was mentioned earlier, photometric measurements using ground-based instruments do not have the required accuracy. While it is clear that HST has the ability to make the needed measurements, it is impractical for such a busy and valuable asset to be committed to a project like this for that length of time. A small satellite like MOST, which could be available to make such observations, would have a difficult time detecting the observed transit signature because its signal would be diluted by the current close proximity of α Centauri A. A larger instrument like NASA’s repurposed Spitzer IR telescope is too sensitive to observe the bright stars of α Centauri and its detectors would be hopelessly saturated. And NASA’s Kepler has no way of observing a star so far from the ecliptic given its current operational limitations. A future space-based telescope dedicated to searching for transits of extrasolar planets will be required to reobserve these transits. Perhaps NASA’s TESS (Transiting Exoplanet Survey Satellite) mission or ESA’s CHEOPS (Characterizing Exoplanets Satellite), which are both scheduled for launches in 2017, can make the required observations.

An artist’s depiction of NASA’s TESS satellite which will systematically search the bright stars of the sky for planetary transits starting in 2017. (NASA)

Conclusion

The attempt to confirm the presence of α Centauri Bb using the independent observing technique of photometry to detect transits has failed to verify this controversial find. This lack of a detection should not be taken as evidence that this extrasolar planet does not exist since there was only a 9.5% probability that its orbit would be oriented to allow detection by this method. We will have to continue our wait for new data before the question of α Centauri Bb is resolved one way or the other.

Frustratingly, this latest attempt to confirm α Centauri Bb has only deepened the mystery of the nature of the planetary system of α Centauri B by finding a transit-like signature of what could be yet another planet in a tight orbit around this nearby Sun-like star. The observation of a single transit in 40 hours of HST data, if it is indeed a transit, only constrains the orbital period of this potential find to being no more than about 20.4 days and likely about 12.4 days. Adding to the frustration is the fact that confirming this potential discovery will not be possible anytime soon. It seems that it might be a few more years before we begin to clear up the current mysteries and have a clearer picture of what planets might orbit α Centauri B.

Follow Drew Ex Machina on Facebook.

Related Reading

“The Search For Planets Around Alpha Centauri”, Drew Ex Machina, August 11, 2014 [Post]

“The Search For Planets Around Alpha Centauri – II ”, Drew Ex Machina, September 25, 2014 [Post]

“Happy Anniversary Alpha Centauri Bb?”, Centauri Dreams, October 16, 2014 [Post]

“The Search For Planets Around Proxima Centauri”, Drew Ex Machina, February 23, 2015 [Post]

General References

Brice-Olivier Demory et al., “Hubble Space Telescope search for the transit of the Earth-mass exoplanet Alpha Centauri Bb”, arXiv 1503.07528 (accepted for publication in Monthly Notices of the Royal Astronomical Society), March 25, 2015 [Preprint]

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

Peter Plavchan, Xi Chen and Garret Pohl, “What is the mass of of α CEN Bb?”, arXiv 1503.01772 (submitted to The Astrophysical Journal), March 5, 2015 [Preprint]