Probing a new class of exoplanet

The mysterious world is unlike anything found in our solar system.

This artist's illustration shows the theoretical internal structure of the exoplanet GJ 3470 b. It is unlike any planet found in the Solar System. Weighing in at 12.6 Earth masses the planet is more massive than Earth but less massive than Neptune. Unlike Neptune, which is 3 billion miles from the Sun, GJ 3470 b may have formed very close to its red dwarf star as a dry, rocky object. It then gravitationally pulled in hydrogen and helium gas from a circumstellar disk to build up a thick atmosphere. The disk dissipated many billions of years ago, and the planet stopped growing. The bottom illustration shows the disk as the system may have looked long ago. Observations by NASA's Hubble and Spitzer space telescopes have chemically analyzed the composition of GJ 3470 b's very clear and deep atmosphere, yielding clues to the planet's origin. Many planets of this mass exist in our galaxy. Credit: NASA, ESA, and L. Hustak
This artist's illustration shows the theoretical internal structure of the exoplanet GJ 3470 b. It is unlike any planet found in the Solar System. Weighing in at 12.6 Earth masses the planet is more massive than Earth but less massive than Neptune. Unlike Neptune, which is 3 billion miles from the Sun, GJ 3470 b may have formed very close to its red dwarf star as a dry, rocky object. It then gravitationally pulled in hydrogen and helium gas from a circumstellar disk to build up a thick atmosphere. The disk dissipated many billions of years ago, and the planet stopped growing. The bottom illustration shows the disk as the system may have looked long ago. Observations by NASA's Hubble and Spitzer space telescopes have chemically analyzed the composition of GJ 3470 b's very clear and deep atmosphere, yielding clues to the planet's origin. Many planets of this mass exist in our galaxy. Credit: NASA, ESA, and L. Hustak

For the first ever time, a team of scientists have analyzed the chemical composition of the atmosphere of a new class of exoplanet, called GJ 3470 b, whose mass is 12.6 times that of Earth, but somewhat less than that of Neptune, which weighs in at 17 Earth masses.

Scientists found that the exoplanet has a rocky core buried under a deep, crushing hydrogen and helium atmosphere. Eighty percent of the planets in our galaxy has similar mass and size range, but astronomers have always been unable to comprehend the chemical nature of the environment of such a planet until now.

Université de Montréal astronomy professor Björn Benneke, who led the study, said, “We expected to find an atmosphere strongly enriched in heavier elements like oxygen and carbon that create abundant water vapor and methane gas. Instead, we found an atmosphere that is so poor in heavy elements that its composition resembles the hydrogen- and helium-rich composition of the sun.”

The most plausible explanation, according to Benneke, is that GJ 3470 b was born precariously close to its red dwarf star, which is about half the mass of our Sun. He hypothesizes that mostly it started as a dry rock, and rapidly accreted hydrogen from a primordial disk of gas when its star was very young. The disk is called a “protoplanetary disk.”

Caltech planetary scientist Heather Knutson said, “We understand the planet better when we observe its atmosphere using more than one technique; this is particularly true for planets with high-altitude clouds, which usually only affect select regions of the atmosphere.”

An analysis of the new planet was published in Nature Astronomy on July 1.