Astronomers discover lowest mass exoplanet seen directly

Of the nearly 2000 exoplanets discovered to date, only about 10 have been seen directly, because they are so faint compared with the bright stars they orbit. Now, an instrument designed for direct imaging has found its first new exoplanet: a Jupiter-like world 100 light-years away in the constellation Eridanus. It is the faintest and least massive exoplanet directly imaged so far, and the first to show an atmosphere rich in methane, similar to the giant planets in our own solar system. “This spectrum really looks like a planet,” says Bruce Macintosh, a physicist at Stanford University in Palo Alto, California, and the principal investigator of the Gemini Planet Imager (GPI), which made the discovery.

The planet, known as 51 Eri b, is quite young—only about 20 million years old. It is roughly twice the mass of Jupiter and orbits more than twice as far from its star than Jupiter does from the sun, the team reports online today in Science. According to Macintosh, astronomers had expected to see strong evidence for methane in the spectra of giant Jupiter-like exoplanets but until now had detected only traces. Many other direct-imaged planets have spectra more like those of small, cool stars, Macintosh says, whereas 51 Eri b shows strong signals for both water vapor and methane. “Since the atmosphere of 51 Eri b is also methane rich, it signifies that this planet is well on its way to becoming a cousin of our own familiar Jupiter," says GPI team member Mark Marley, an astrophysicist at NASA’s Ames Research Center in Mountain View, California. Macintosh agrees. “It’s really the most Jupiter-ish thing ever imaged directly,” he says.

Almost all exoplanets found so far have revealed themselves indirectly, either by making their parent stars wobble back and forth with each swing of their orbit, or by dimming them as the planets pass in front. Those techniques tell astronomers about a planet’s mass or size but little about what it’s like. Splitting a planet’s own light into a spectrum can reveal something about the chemistry of its atmosphere. But capturing that light directly is like trying to spot a firefly close to the beam of a searchlight. Today’s telescopes just don’t have the resolution to do it, except in a few cases in which the star is dim and the planet is bright and moves in a wide orbit.

51 Eri b is the first previously unknown planet found by a new generation of instruments designed to increase the number of direct sightings. These are sophisticated boxes of optics attached to some of the world’s biggest ground-based telescopes. GPI is fixed to the 8-meter Gemini South telescope on Chile’s Cerro Pachón, whereas its rival Spectro-Polarimetric High-contrast Exoplanet REsearch uses the European Southern Observatory’s Very Large Telescope at Cerro Paranal in Chile. Others include Project 1640 on the Palomar 200-inch telescope in California and Subaru Coronagraphic Extreme Adaptive Optics on Japan’s 8-meter Subaru Telescope on Mauna Kea, Hawaii.

All these instruments use a coronagraph—a mask to block out the light from the parent star—plus complex optics to remove stray light diffracted off the edge of the coronagraph. They also employ extreme adaptive optics: mirrors that change shape rapidly to compensate in real time for the way Earth’s atmosphere distorts the planet’s light.

Despite the sophistication, direct imaging remains so difficult that astronomers will be able to see only large exoplanets—Jupiter-sized or bigger—that are young, still hot, and glowing with infrared light. But such planets are valuable to researchers trying to figure out how planets form from the disks of debris and gas surrounding young stars. The best examples astronomers can study at the moment are the planets in our own solar system, which have been settling down for billions of years. Seeing young exoplanets at various stages of evolution will enable researchers to test rival models of planet formation. 51 Eri b, which is glowing at about 650 K, “is exactly the kind of system we envisioned discovering when we designed GPI,” says GPI project scientist James Graham of the University of California, Berkeley.

“It’s great to see GPI found something interesting so soon,” says planetary scientist Sara Seager of the Massachusetts Institute of Technology in Cambridge, Massachusetts. “Hopefully this will be the start of a long line of results from direct imaging.”

Posted in Space