News | October 16, 2019

Discovery Alert: 2-planet System Is Close – and Weird

By Pat Brennan

Artist's rendering of a Neptune-type exoplanet in the icy outer reaches of its star system. It could look something like a large, newly discovered gas giant that takes about 20 years to orbit a star 11 light-years away from Earth; the discovery of this world, and confirmation of a sister planet, makes this the closest known multi-planet system to our own. Image credit: NASA's Goddard Space Flight Center/Francis Reddy

Planets: GJ 15 A b and c

The discovery: GJ 15 A b and c orbit a red-dwarf star just 11 light-years away, making them our nearest multi-planet neighbors – at least among the exoplanet systems discovered so far.

Date: Entered into NASA's Exoplanet Archive on October 10, 2019

Key facts: These planets have an abundance of strange properties. The innermost world, GJ 15 A b, takes just 11 days to make a full orbit around the star – a "year" on this planet. It weighs in at three times the mass of Earth, qualifying it for the size category known as "super Earth." It's also super-heated, with an estimated surface temperature of 530 degrees Fahrenheit (276 Celsius).

Its sister-world, GJ 15 A c, could hardly be more different. It's a gas giant 36 times the mass of Earth, and appears to have roughly a 20-year orbit around its cool and small parent star. While its temperature is not known, it's likely to be quite cold in this distant, possibly icy orbit, loosely comparable to the orbit of Saturn around our Sun.

Details: The orbit of at least the outer world might well have been sculpted by their parent star's distant companion, another red dwarf known as GJ 15 B, although more observations are needed to be sure.

Fun facts: GJ 15 A b is an on-again, off-again planet. It was first reported in 2014, then "refuted" after another team of astronomers could not detect it. The most recent paper "reinstates" the planet with more data backing up the original discovery of a super Earth in an 11-day orbit. The two planets are among a batch of 16 added on October 10 to NASA's Exoplanet Archive, bringing the total of confirmed exoplanets in our galaxy to 4,073.

The discoverers: The new discovery, planet c, and the confirmation of planet b were announced in a 2018 paper from a team led by Matteo Pinamonti using the Italian Telescopio Nazionale Galileo, along with archival data. The team relied on the "wobble" method of planet detection, or radial velocity: measuring the subtle back-and-forth motion of a star as it is tugged one way, then another, by the gravity of an orbiting planet. A team led by Andrew Howard of the University of Hawaii made the original discovery of planet b in 2014.