First-Time Detection of Visible Light Reflected off an Exoplanet, 51 Pegasi b

April 26, 2015

After being only the third exoplanet discovered and the first one around a Sun-like star, 51 Pegasi b is now the first planet to be observable by the visible light reflected off it.

51 Pegasi bArtist's impression of 51 Pegasi b. Credit: ESO

In the constellation Pegasus, some 50 light-years away from Earth, lies a Jupiter-like gas giant named 51 Pegasi b. In 1995, it was the first exoplanet discovered around a star that is similar to our Sun. It orbits its star at a distance of 0.05 astronomical units, or merely 7.5 million kilometres, which make the planet very hot and reflective. 51 Pegasi b was the first of many so-called hot Jupiters to be discovered. A hot Jupiter is a gas giant similar to Jupiter in mass and size, but orbiting its star at a much closer distance.

51 Pegasi b was discovered using radial velocity method i.e. using a sensitive spectroscope to detect slight velocity changes in the star’s spectral lines. These changes happen regularly, which is a sign that a planet is orbiting a star and the planet’s gravity is what causes these changes.

Almost twenty years later, astronomers were able to pick up the visible light reflected off the surface of the 51 Pegasi b. This is the first planet to be detected using this technique!

Methods usually used to detect exoplanets (at the moment 1900 confirmed discoveries) are all based on planet’s transit in front of the star, as viewed from Earth. Then, the change in the light sent by the star is observed to determine planet’s atmospheric composition, pressure, temperature and scale height, which allows for calculation of planet’s mass.

Since 51 Pegasi b does not transit in front of its star, 51 Pegasi, these transit-based methods were useless to find out more about the planet. However, using ESO’s High Accuracy Radial velocity Planet Searcher (HARPS), a spectrograph at La Silla 3.6m telescope in Chile, a team led by Jorge Martins, from Instituto de Astrofísica e Ciências do Espaço at Universidade do Porto, Portugal, was able to detect a visible light that reflected off the planet’s surface.

Needless to say, it is very difficult to detect the light from the planet, because it is out-shined by the star it orbits, especially if it orbits the star at such close distance. Nevertheless, the team used the star’s spectrum as a template for what to look for and managed to find a similar signature that reflected off the 51 Pegasi b.

"This type of detection technique is of great scientific importance, as it allows us to measure the planet’s real mass and orbital inclination, which is essential to more fully understand the system," said Martins. "It also allows us to estimate the planet’s reflectivity, or albedo, which can be used to infer the composition of both the planet’s surface and atmosphere."

The data the team gathered suggests that the 51 Pegasi b is almost twice the size of Jupiter, but it only has half the mass. The results still await for confirmation from an independent, second team, but the implications for the future of the search for exoplanets are tremendous.

ESO’s 3.6 m telescope has limited application for this technique, but the fact the discovery was possible allows for the optimism when future spectrographs begin to watch the skies and search for non-transit exoplanets in our neighbourhood. One of these future spectrographs is Echelle SPectrograph for Rocky Exoplanet- and Stable Spectroscopic Observations (ESPRESSO) at ESO’s Very Large Telescope (VLT), which should begin with operations the next year. Even better results should be achieved with European Extremely Large Telescope (E-ELT), which is under construction since 2014 and should be finished by 2024.

"We are now eagerly awaiting first light of the ESPRESSO spectrograph on the VLT so that we can do more detailed studies of this and other planetary systems," added Nuno Santos, of the IA and Universidade do Porto, co-author of the new paper that was published on April 22, 2015 in the journal Astronomy & Astrophysics.

Source ESO, Source A&A

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