An artist's impression of the exoplanet 51 Pegasi b obtained following detailed information on the first exoplanet detected ESO

Astronomers have made the first direct observation of an exoplanet's visible light spectrum in a technique that promises to accelerate the number of such worlds studied, especially with the advent of new generation instruments.

The observations were made on a Jupiter-sized planet 51 Pegasi b, the first exoplanet to be discovered in 1995, at a distance of 50 light years from Earth in the constellation Pegasus.

The new method reveals 51 Pegasi b has a mass about half that of Jupiter's and an orbit with an inclination of about nine degrees to the direction to the Earth.

The planet also seems to be larger than Jupiter in diameter and to be highly reflective, as expected for a hot Jupiter that is very close to its parent star and exposed to intense starlight.

Led by Jorge Martins from the Instituto de Astrofísica e Ciências do Espaço (IA) and the Universidade do Porto, currently a PhD student at ESO in Chile the team used the HARPS (High-Accuracy Radial velocity Planet Searcher) instrument on the ESO 3.6-metre telescope at the La Silla Observatory in Chile.

The new technique allows the planetary spectrum to be directly detected in visible light, allowing access to different characteristics of the planet. As it does not depend on finding a planetary transit, it can potentially be used to study many more exoplanets.

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.

Light from the star when it falls on the planet's atmosphere will be reflected in the different wavelengths according to absorption by gases in the atmosphere.

Jorge Martins explains: "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. 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."

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.

The other approach is to observe the system when the star passes in front of the planet, which primarily provides information about the exoplanet's temperature.

This method, known as transit spectroscopy, is restricted in use to times when stars and planets align.

The exoplanet 51 Pegasi b was discovered in 1995 and was the first confirmed exoplanet to be found orbiting an ordinary star like the Sun.

It is among a growing class of exoplanets similar in size and mass to Jupiter but orbiting their stars in closer orbits.

Since 51 Pegasi b, almost 1900 exoplanets in 1200 planetary systems have been confirmed.
Scientists have used data from Nasa's Kepler satellite to work out that billions of stars in the Milky Way will have one to three planets in the habitable zone.

Many stars have systems with two to six planets, but there could be far more unobservable with Kepler.

The European Space Agency's Gaia satellite has also been involved in the search for exoplanets, by looking at the parent star's wobbles caused by the orbiting planet.

Nasa's Wide-field Infrared Survey Explorer (WISE) recently scanned the skies for signs of an advanced civilisation.

More projects to be launched in the coming decade will strengthen the search for exoplanets. Among these are the Wide-Field Infrared Survey Telescope (WFIRST), James Webb Space Telescope (JWST), Transiting Exoplanet Survey Satellite.

ESO's Very Large Telescope and the future European Extremely Large Telescope are some of the ground-based astronomy projects that will help clear the picture.