A planet with three stars? Sounds exciting, right? It is, but it’s the way scientists found it that is pretty damn cool. They directly imaged it. Scientists have done this before, but it’s rare.

Usually, when we hear about newly discovered exoplanets, scientists use the transit method.

They wait for the planet to pass in front of its host star and then watch to see how much light is blocked by it. Using that measurement along with how long it takes to pass in front of the star, we can figure out how big the planet is and its orbital period.

In this case, a team of astronomers from the University of Arizona pointed ESO’s Very Large Telescope at HD 131399Ab. And they found it sits in a three-star system. Check out the planet and its three stars below.

direct image exoplanet

Ok, a couple of things about that image. It’s a composite of two images. One shows the three stars. The other shows the exoplanet (HD 131399Ab). While the planet appears bright compared to the stars, it isn’t.

“HD 131399Ab is one of the few exoplanets that have been directly imaged, and it’s the first one in such an interesting dynamical configuration,” said Daniel Apai. He leads the research group tasked with finding and observing exoplanets at the University of Arizona.

A job for SPHERE

SPHERE instrument

The SPHERE instrument.

But, how could astronomers directly image this exoplanet? They used an instrument on ESO’s Very Large Telescope called SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch).

This instrument is specifically designed to look directly at exoplanets. Because it’s sensitive to infrared light, SPHERE is great for detecting the heat signature of young exoplanets. And with HD 131399Ab being just 16 million years old, it’s throwing off plenty of heat. Scientists estimate the exoplanet has a temperature of around 580 degrees Celsius. That’s warm, but on the cool side for directly-imaged exoplanets.

What about starlight? Wouldn’t the proximity of an exoplanet to its star make it difficult to see directly? SPHERE uses a tool commonly used to study the outer layers of the sun, a coronagraph.

Juan Carlos Muñoz, an ESO astronomer, talked a bit more about how SPHERE isolates the light astronomers want to see. “The polarimetric differential imaging mode of SPHERE works on this principle: the light emitted by the central star is unpolarised, but the light scattered by the dusty disc is polarised, so we can use this difference to isolate one from the other and get a very sharp view of the disc itself.”

HD 131399Ab is about four Jupiters

The team of astronomers estimate the exoplanet is about four Jupiter masses. And it’s located 320 light-years from Earth in the constellation of Centaurus.

More observations will be needed to determine the exact orbit of the planet, but here’s what the astronomers have so far.

At the center of this triple-star system lies HD 131399A. About 300 AU away (one AU equals the distance between the sun and Earth) sits two smaller stars, B and C. These two stars are separated by just 10 AU. If this scenario is right, the exoplanet orbits star A at about 80 AU.

“If the planet was further away from the most massive star in the system, it would be kicked out of the system,” Apai explained. “Our computer simulations have shown that this type of orbit can be stable, but if you change things around just a little bit, it can become unstable very quickly.”

exoplanet three star orbits

Multi-star systems might seem foreign to us, but they are just as common as single star systems. Kevin Wagner, the paper’s author, explains why it’s important to continue studying HD 131399Ab.

“It is not clear how this planet ended up on its wide orbit in this extreme system, and we can’t say yet what this means for our broader understanding of the types of planetary systems, but it shows that there is more variety out there than many would have deemed possible,” says Wagner. “What we do know is that planets in multi-star systems have been studied far less often, but are potentially just as numerous as planets in single-star systems.”


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