‘Hot Jupiter’: Scientists catch light bouncing off an exoplanet

An artist's impression shows the exoplanet 51 Pegasi b, sometimes referred to as Bellerophon, orbiting a star about 50 light-years from Earth in the constellation Pegasus.

An artist’s impression shows the exoplanet 51 Pegasi b, sometimes referred to as Bellerophon, orbiting a star about 50 light-years from Earth in the constellation Pegasus.

(M. Kornmesser, Nick Risinger / ESO )

An international team of astronomers says it has managed to take the first direct visible-light spectrum from an exoplanet, providing a new tool to probe the nature of the “hot Jupiter” known as 51 Pegasi b.

The findings, published in the journal Astronomy & Astrophysics, offers a promising way forward to study exoplanets that doesn’t rely on waiting for a distant planet to transit, or pass in front of its host star.

“I was really happy. It was awesome ... especially when I started looking at the implications of it,” said lead author Jorge Martins, an astronomer at the University of Porto in Portugal working on his PhD while at the European Southern Observatory in Chile, known as ESO. “It’s like, we’re observing a planet that is at a distance of like 3 million times the distance of the Earth to the sun, and we’re still able to see the reflected light of the planet. I don’t know. I think it’s overwhelming.”

51 Pegasi b has long been known as the first confirmed discovery of a planet around a sun-like star. The planet, whose star sits about 50 light-years away in the constellation Pegasus, was found in 1995 and is seen as a typical “hot Jupiter” -- the kind of gassy heavyweight planet that is in the same size class as Jupiter but that orbits extremely close to its home star.


Since 51 Pegasi b’s discovery two decades ago, more than 1,900 exoplanets -- planets beyond our solar system -- have been discovered, but relatively little is known about these distant worlds. Much of what we do know about an exoplanet’s atmosphere, temperature and radius is gathered by studying the planet as it transits across the surface of its star, blocking a little bit of starlight and allowing a tiny amount to pass through its thin atmosphere. By watching how the planet’s gravity tugs slightly on the star, scientists can learn something about its orbit and its mass. But direct measurements of reflected light would give astronomers a serious leg up, Martins said.

Using the HARPS instrument at ESO’s La Silla Observatory, the researchers took the particular fingerprint of light — or spectrum — coming from the star, and were able to look for the faint reflection of this fingerprint that would be bounced back by the exoplanet.

This is an extremely painstaking process requiring exquisite precision, Martins added -- and they could only observe for a few nights a year because of several limitations, including that the planet had to be in just the right position relative to its star and to Earth in order to catch the tiny amount of reflected light.

Using the data, they were able to place constraints on the planet’s mass, finding that it was about half the mass of Jupiter (0.46 of a Jovian mass, give or take) and an orbital inclination of about 80 degrees.


“This result is encouraging and constitutes a very valuable proof of concept,” the study authors wrote.

The fact that the scientists were able to pick out this weak signal using the HARPS data tells them that with the advent of much stronger instruments, astronomers will be able to pick out the light coming from even more exoplanets, the authors wrote. Among the instruments to look forward to using: the ESPRESSO instrument at the ESO’s Very Large Telescope as well as the European Extremely Large Telescope at the site.

“These encouraging results clearly show a bright future for this type of studies when next-generation instruments (e.g., ESPRESSO at the VLT) and telescopes (e.g., ESO’s E-ELT) become available to the community,” the researchers wrote. “The sheer increase in precision and collecting power will allow for the detection of reflected light from smaller planets, planets on orbits with longer periods, or an increase in detail for larger planets like 51 Peg b.”

Using such instruments, scientists might be able to observe the light bouncing off not just close-in hot Jupiters, but smaller, farther-off exoplanets -- perhaps even Earth-sized planets in their stars’ habitable zones.


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