[LINK] "Has Another Planet Been Found Orbiting Alpha Centauri B?"

[rfmcdonald]

Drew Ex Machina's Andrew LePage has a great analysis of a recent attempt by astronomers to confirm the existence of close-orbiting Alpha Centauri Bb that may have accidentally discovered another planet orbiting Alpha Centauri B somewhat further out. (Alpha Centauri Bc, I presume it would be named.)

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.