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Astronomers get first direct glimpse of exoplanet's light

Paul Sutherland, Feature writer
Apr 22, 2015, 19:08 UTC, Updated Apr 23, 2015, 0:33 UTC

Sen—Astronomers have used a planet-seeking device to make the first direct observation of the spectrum of visible light reflected from an exoplanet.

They detected the host star's light shining from an alien world labelled 51 Peg b. This was the very first exoplanet to be discovered around an ordinary star, designated 51 Peg, back in 1995, using the radial velocity technique that measures how the planet makes the star wobble.

The new discovery is exciting for scientists because it allows them to determine some of the planet’s physical properties, such as its mass, the nature of its atmosphere and the tilt of its orbit. 

They find that it is larger than Jupiter, but only about half as massive—typical for a so-called “hot Jupiter”—and its orbit is inclined by about 9° to the direction of Earth, so close to being viewed edge-on.

The breakthrough was made by a team led by PhD student Jorge Martins from the Instituto de Astrofísica e Ciências do Espaço (IA) and the Universidade do Porto, Portugal, who is currently working at the European Southern Observatory in Chile. 

He used the HARPS (High Accuracy Radial velocity Planet Searcher) instrument on the ESO 3.6-metre telescope at the La Silla Observatory in Chile to watch the star, which lies at a relatively close distance of about 50 light-years, in the constellation of Pegasus. 

One of the co-discoverers of the planet in 1995 was quick to offer his congratulations on the new direct-light observation by Martins’ team. 

Professor Didier Queloz, who is now at the Battcock Centre for Experimental Astrophysics, in Cambridge, England, told Sen: “This is a wonderful present at the occasion of the 20 years of 51 Peg’s discovery. I congratulate the team for this achievement. 

“The measurement is very challenging. It provide serious hints about the structure of the planet atmosphere and nails down its mass.”

Previously, the main way to study an exoplanet’s atmosphere was to observe one with a spectroscope as it passed in transit, across the face of its host star, then compare the readings with that when the star alone was examined.

The new technique again uses the spectrum of the star as a template, from which the signature of reflected light off a planet can be found. If it sounds easy, it is not, because the planet’s light is exceedingly faint compared to the star. But it offers great potential because it allows planets to be studied which do not pass directly in front of their stars.

Jorge Martins told Sen: “Since this is a direct observation of the reflected spectrum of the planet, we can infer almost immediately the planet’s orbital inclination and real mass, essential to fully characterise the planetary orbit. Furthermore it were able to estimate the planet’s reflectivity which suggests that the planet is an inflated hot-Jupiter planet, with a high reflectivity. 

“Please note that this is the first application of our technique in visible light, and additional observations will permit us to better characterise the planet, as at this moment this was more like a proof of concept.”

Because the new observation was made with a 3.6-metre telescope, astronomers are looking forward to the results when they use the technique with much larger instruments, such as the 8.4-metre Very Large Telescope with ESPRESSO (the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations), in Chile, or the forthcoming European Extremely Large Telescope to be built nearby.