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Rare super-Neptune exoplanet spotted in nearby binary system

An artist’s impression of two exoplanets orbiting a pair of red dwarfs. NASA / JPL-Caltech / T. Pyle /

Astronomers using the Italian Telescopio Nazionale Galileo and the High Resolution Echelle Spectrometer on the Keck Telescope have discovered a rare type of exoplanet called a super-Neptune, located rather close by at a distance of just 11.6 light-years from Earth. This type of planet is larger than Neptune and around five to seven times the size of Earth. This is different than the type of exoplanets typically located by our current tools, which are usually super-Earths or gas giants similar to Jupiter or Saturn.

This particular super-Neptune is located in the Gliese 15 binary system, in which planets move around two red dwarf stars. Astronomers first became interested in this system in 2014, when they located a super-Earth orbiting around the larger star, called Gliese 15A. This super-Earth, named Gliese 15Ab, comes very close to the star with an orbital period of just 11.44 days, so it has extremely high surface temperatures of 530 degrees Fahrenheit (276 degrees Celsius).

Now, astronomers have spotted a second planet in orbit around Gliese 15A, which has a mass that is 36 times that of the Earth.

The team discovered the planet using a method called radial velocity or Doppler spectroscopy. This method analyzes tiny movements of stars, as the stars are affected by the planets that move around them. Even though the planets are much smaller than the stars, they still exert a gravitational force on the bodies around which they move. Astronomers can look for evidence of these small wobbles of stars to deduce the existence of an exoplanet.

Using a tool called a spectrometer that measures light in a specific band of the electromagnetic spectrum, astronomers can see how light from a star shifts toward the blue or red side of the spectrum due to the Doppler effect. If there is a regular cycle of redshifting to blueshifting, that suggests the star is moving back and forth, which in turn suggests the presence of a planet.

“With its period of 7,600 days (20.8 years), Gliese 15Ac is the longest-period sub-Jovian planet detected up to date with the radial velocity method,” Dr. Pinamonti and colleagues wrote in their paper. “With the confirmed presence of two widely spaced planetary-mass companions, Gliese 15A is now the multi-planet system closest to our sun.”

The findings are published in the journal Astronomy & Astrophysics.

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