The exoplanet 51 Pegasi b detected in reflected visible light

Wide-field view of the sky around the star 51 Pegasi
Image and further reading: ESO Text: A&A

Twenty years ago, the discovery of 51 Peg b heralded the birth of a new field, exoplanetology. However, the Jupiter-like planet that orbits at only 0.05 AU from its parent star does not transit in front of its star. The emphasis therefore shifted toward the study of other hot Jupiters transiting bright stars, for which sizes and spectra can be obtained. Martins et al. take a new look at the planet with the spectrograph HARPS and provide evidence of reflected light from the planet. In order to do so, they cross-correlate the spectrum of the star Doppler-shifted by the motion of the planet on its orbit. This provides two pieces of information: the maximum Doppler shift of the planetary spectral lines is linked to the inclination of the orbit; and the amplitude of the lines compared to those of the star measures the ratio of the planetary to the stellar flux. They find that the orbit is instead seen edge-on with an inclination of 61° to 90°, meaning that the true mass of 51 Peg b is 0.45 to 0.52 MJup. They also find a surprisingly high flux ratio of about 60 ppm (parts per million), which would imply that 51 Peg b has a large visible albedo and a large radius (albedo of 0.5 and radius of 1.9 RJup). While this is within the realm of possibilities, it begs for an independent confirmation of this difficult measurement. But the study does shows that today’s spectrographs have reached a sensitivity and precision that enables the observation of even non-transiting planets, paving the way for characterizing the population of close-in planets in our neighborhood.

The team that made this new detection was led by Jorge Martins from the Instituto de Astrofísica e Ciências do Espaço (IA) and the Universidade do Porto, Portugal, who is currently a PhD student at ESO in Chile. Needless to say, this is the first detection of an exoplanet in reflected light and it is perhaps fitting that the target is the first exoplanet to be discovered by the radial velocity method, as long ago as 1995. The paper is Martins et al., “Evidence for a spectroscopic direct detection of reflected light from 51 Pegasi b”, A&A 576, A134 (2015).

Update: The Automated Planet Finder (APF) plays an important role in the discovery of two new “super-Earth” exoplanets in orbit around the K0.5V star HD 7924 (arXiv pdf)

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