For the first time, astronomers have directly detected visible light reflected off an extrasolar planet.
This artist’s view shows the hot Jupiter exoplanet 51 Pegasi b. Image credit: ESO / M. Kornmesser / Nick Risinger, skysurvey.org.
The first exoplanet ever discovered around a normal star, 51 Pegasi b, is located in the northern constellation Pegasus, roughly 50 light-years away.
It was discovered in 1995 and is regarded as the archetypal hot Jupiter – a class of exoplanets now known to be relatively commonplace, which are similar in size and mass to Jupiter, but orbit much closer to their parent stars.
Now, a team of astronomers led by Dr Jorge Martins from the Instituto de Astrofísica e Ciências do Espaço and the Universidade do Porto, Portugal, has used the HARPS instrument on the ESO 3.6-m telescope at the La Silla Observatory in Chile to directly detect 51 Pegasi b spectroscopically in visible light.
“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 Dr Martins, who is the lead author on the study published in the journal Astronomy & Astrophysics.
“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.”
This image shows the sky around the star 51 Pegasi. Image credit: ESO / Digitized Sky Survey 2.
51 Pegasi b was found to have a mass about half that of Jupiter’s and an orbit with an inclination of about 9 degrees to the direction to the Earth.
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 method 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.
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J. H. C. Martins et al. 2015. Evidence for a spectroscopic direct detection of reflected light from 51 Pegasi b. A&A, vol. 576, A134; doi: 10.1051/0004-6361/201425298
