A planet 100 light-years away that resembles a young Jupiter has been discovered by an international team of astronomers that includes six UCLA scientists. The discovery is reported Aug. 13 in the online edition of the journal Science.

The planet, called 51 Eridani b, is only 20 million years old — a mere infant by astronomy standards. (Jupiter, the sun and Earth are all about 4.5 billion years old.) It is the first planet detected by the Gemini Planet Imager, or GPI, which was designed to discover and analyze faint, young planets orbiting bright, nearby stars.

The vast majority of solar systems that have been discovered are very different from our own, with massive planets close to their stars, said co-author James Larkin, a UCLA professor of physics and astronomy.

“Previous search methods couldn’t find systems like our own, with small, rocky worlds close to their star and large, gas giants at large distances like Jupiter and Saturn,” Larkin said. “The search for large planets at large separations from their star is exactly the goal of GPI. These solar systems are likely much more similar to our own. GPI will reveal to us how common our solar system architecture truly is.”

Larkin and colleagues at UCLA’s Infrared Laboratory for Astrophysics developed and built GPI’s highly advanced spectrometer, which enabled the instrument to detect the presence of methane on 51 Eridani b. It revealed that the planet has the strongest concentration of methane ever detected on a planet outside the Milky Way — as well as the presence of water — which indicates that it’s similar to planets in our solar system and should yield additional clues about how the planet formed.

The spectrograph produces 18 images at different wavelengths of light, which enables GPI to reject light from nearby stars, which can be up to 10 million times brighter than the planets being studied. The light from 51 Eridani b is very faint; its nearest star is 3 million times brighter.

Michael Fitzgerald, UCLA associate professor of physics and astronomy, is a co-author of the Science paper and a lead co-investigator of GPI’s survey team. Other UCLA co-authors are Benjamin Zuckerman, a professor of physics and astronomy; and graduate students Thomas Esposito, Li-Wei Hung and Pauline Arriaga.

The project’s lead investigator is Bruce Macintosh, who received his doctorate from UCLA in 1995, working in the infrared laboratory, and who is now a professor of physics at Stanford University’s Kavli Institute.

“Many of the exoplanets astronomers have imaged before have atmospheres that look like very cool stars,” Macintosh said. “This one looks like a planet.”

(An exoplanet is a planet outside our solar system.)

GPI was installed on the 8-meter Gemini South Telescope in Chile in 2013, and its science operation began in 2014. The GPI team has studied almost 100 stars already.

The newly discovered planet orbits a little farther from its parent star than Saturn does from the sun. It is roughly twice the mass of Jupiter. Until now, the gas giant planets that have been directly detected have been much larger — five to 13 times Jupiter’s mass, Fitzgerald said.

The scientists reported that 51 Eridani b has a temperature of 800 degrees Fahrenheit, hot enough to melt lead, but still rather cold compared with other gas giants, which reach temperatures above 1,000 degrees. Previous Jupiter-like exoplanets have shown only faint traces of methane, far different from the heavy methane atmospheres of the gas giants in our solar system.

“This is exactly the kind of planet we envisioned discovering when we designed GPI,” said James Graham, a UC Berkeley professor and GPI’s project scientist.

GPI will provide new insights about how solar systems form. Astronomers believe that the gas giants in our solar system formed by building up a large core over a few million years and then pulling in a huge amount of hydrogen and other gasses to form an atmosphere. But the Jupiter-like exoplanets that have been discovered so far are much hotter than models have predicted, hinting that they could have formed much faster as material collapses quickly to make a very hot planet. The core-buildup process can also form rocky planets like the Earth, while the process of fast-collapsing materials might make only giant gas planets, the researchers said. 51 Eridani b is young enough to reveal clues about how it was created.

“This planet really could have formed the same way Jupiter did; the whole solar system could be a lot like ours,” Macintosh said.

There are hundreds of planets a little bigger than Earth, but so far no way to know if they are “super-Earths,” or micro-sized gas and ice planets like Neptune, or something different altogether, he said.