In 1995, astronomers found the first exoplanet known to science. The planet 51 Pegasi b [1] is 50 light-years away and is classified as a “hot Jupiter”, meaning that it is a gas giant, similar in size to Jupiter but orbiting closer to its host star.

Now, 20 years later, that same planet has been re-discovered. That doesn’t mean that it was ever lost, only that it was detected again using a new method. The technique involves comparing light reflecting off the planets surface to the light of its star. This is the first time that visible light has been used as a detection method and could lead to the detection of new planets as well as new information about known planets.

Currently “transmission spectroscopy” is generally used to examine the atmospheres of exoplanets. The technique involves watching a planet transit across its host star and then examining the light as it passes through the planet’s atmosphere.

The new technique doesn’t require finding a planet’s transit, the planetary spectrum is directly detected in visible light. This allows for the study of aspects of the planet which cannot be detected using infrared.

“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,” said Jorge Martins of the the Instituto de Astrofísica e Ciências do Espaço (IA) and the Universidade do Porto in Portugal in a statement.

Using the new technique, the researchers determined that 51 Pegasi b has a mass of only about half of Jupiter’s. Despite this it appears to have a larger diameter than Jupiter. They also found that the planet’s orbit shifts about 9 degrees in the direction of Earth.

The team used the High Accuracy Radial velocity Planet Searcher (HARPS) instrument on the ESO 3.6-metre telescope in Chile to conduct the research. They also report, however, that the telescope has limited applications for the new technique. They think that the ESO’s Very Large Telescope and the European Extremely Large Telescope, which is expected to come online in 2024, will yield even better results.

“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,” says Nuno Santos, of the IA and Universidade do Porto, who is a co-author a paper published in the journal Astronomy & Astrophysics and is available through the ESO website.

To date there are 1900 confirmed exoplanets in 1200 solar systems with new planets being added to the list on an almost weekly basis. New telescopes, including NASA’s James Webb Space Telescope launching in 2018 are expected to accelerate the discovery process.