Exoplanet Found in Triple Star System

Astronomers have discovered a giant planet with an exceptionally wide orbit in a young system of three suns.

planet in triple-star system

This composite image shows the newly discovered exoplanet HD 131399Ab in the triple-star system HD 131399. The image combines two: one of the planet (which in reality is 1/100,000 as bright as star A) and a second of the stars. The SPHERE imager on the ESO Very Large Telescope in Chile took the images. HD 131399Ab likely orbits star A. At only 16 million years old, the planet likely doesn't inhabit a stable orbit.

ESO / K. Wagner et al.

Today in Science, Kevin Wagner (University of Arizona) and others report the discovery of a gas giant in the triple-star system HD 131399. The planet, HD 131399Ab, appears to orbit the brightest star of the bunch, HD 131399A, which is roughly 10 times brighter than the Sun. The team can’t quite pin down the planet’s path, but the researchers estimate that the world is about 82 astronomical units (or Earth-Sun distances) from the star and could complete an orbit in 400 to 700 years.

HD 131399Ab is one of maybe a half dozen worlds found in triple-star systems. What’s interesting about this exoplanet in particular is that it has the widest orbit of any planet in a multi-star family. In fact, it’s a fair fraction of the way to the pair of much smaller, yellower stars that whirl around the primary at a distance of about 300 a.u.

Wagner’s team found HD 131399Ab as part of a campaign with the Spectro-Polarimetric High-Contrast Exoplanet Research (SPHERE) instrument on the Very Large Telescope in Chile. The team is looking at about 100 young stars — both single and multiples — as part of an effort to better understand how common giant planets in large orbits are.

The researchers directly imaged the exoplanet, a technique used rarely because it’s hard to see a planet hidden in the brilliant glare of its host star: HD 131399Ab is only 1100,000 as bright as star A. But because the system is only 16 million years old, the planet is still glowing with the heat of its formation. The team used the glow to estimate its mass at about 4 times that of Jupiter — so HD 131399Ab is definitely a planet and not a brown dwarf. Spectra reveal methane and water in the exoplanet’s atmosphere, as expected for such objects.

Because the team knows so little about the system’s configuration, it’s unclear whether HD 131399Ab’s wide orbit is stable. The team speculates that the world could have landed itself in its current predicament in one of three ways:

  1. There’s another, unseen planet that kicked HD 131399Ab out from an earlier, closer orbit around the primary star;
  2. HD 131399Ab originally formed around the tight binary in this triplet but was ejected, either by those stars or a planet; or
  3. The whole system has changed with time, and nothing is where it started.

Astronomers haven’t spent much time looking for exoplanets around binary stars because they’re really hard to detect. But multi-star systems are about as common as single stars, so we need to know how often planets can form in these environments to really understand planet formation.

*   *   *

You can read more about the result in the European Southern Observatory’s press release — but a word of caution about the “triple sunrises and sunsets” it describes: the stars B and C lie roughly six times farther from HD 131399Ab than Pluto does from the Sun. The brighter of the two (B) is nearly the same mass and temperature as our star; C is 60% as massive. So the pair would be a few hundredths as bright as the Sun as seen from Pluto. Thus, while the stars would be notable in HD 131399Ab’s skies, the “daylight” they created would be a weak one — maybe 150,000 or less that on Earth. (I’m using ballpark numbers, don't go quoting me.)

Reference: K. Wagner et al. “Direct imaging discovery of a Jovian exoplanet within a triple-star system.” Science. Published online July 7, 2016.

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