Astronomers using the HARPS planet-hunting machine at European Southern Observatory's (ESO) La Silla Observatory in Chile have made the first-ever direct detection of the spectrum of visible light reflected off an exoplanet. These observations also revealed new properties of this famous object, the first exoplanet ever discovered around a normal star: 51 Pegasi b. The result promises an exciting future for this technique, particularly with the advent of next generation instruments, such as ESPRESSO, on the Very Large Telescope (VLT), and future telescopes, such as the E-ELT.
The exoplanet 51 Pegasi b lies some 50 light-years from Earth in the constellation of Pegasus. It was discovered in 1995 and was the first confirmed exoplanet to be found orbiting an ordinary star like the sun. It is also regarded as the archetypal hot Jupiter - a class of planets now known to be relatively commonplace, which are similar in size and mass to Jupiter, but orbit much closer to their parent stars.
Since that discovery, more than 1900 exoplanets in 1200 planetary systems have been confirmed. 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, in Portugal, who is currently a PhD student at ESO in Chile.
Currently, the most widely used method to examine an exoplanet's atmosphere is to observe the host star's spectrum as it is filtered through the planet's atmosphere during transit - a technique known as transmission spectroscopy. An alternative approach is to observe the system when the star passes in front of the planet, which primarily provides information about the exoplanet's temperature.
The new technique does not depend on finding a planetary transit, and so can potentially be used to study many more exoplanets. It allows the planetary spectrum to be directly detected in visible light, which means that different characteristics of the planet that are inaccessible to other techniques can be inferred.
The host star's spectrum is used as a template to guide a search for a similar signature of light that is expected to be reflected off the planet as it describes its orbit. This is a difficult task as planets are dim in comparison to their parent stars. The signal from the planet is also easily swamped by other tiny effects and sources of noise.
"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," Martins said, according to a press release. "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."
"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," said co-author Nuno Santos.
This research was presented in a paper "Evidence for a spectroscopic direct detection of reflected light from 51 Peg b", by J. Martins et al., to appear in the journal Astronomy & Astrophysics on April 22.
