Planet discovered in extreme system with three suns

The planet experiences either constant daylight or triple sunrises and sunsets depending on the seasons, which last longer than a human lifetime

In a study published in Science today, researchers have discovered a new planet 340 light years from Earth in a unique three-star system. We talked to the leader of the research group, Daniel Apai from the University of Arizona, to find out what the discovery means for what we know about our universe.

ResearchGate:  Could you explain the significance of your team’s recent discovery of planet HD 131399Ab?

Daniel Apai: This discovery is exciting for three reasons: Although we know of many exoplanets by now, almost all them are indirectly detected, making it difficult to study their atmospheres in detail. In contrast, we discovered HD 131399Ab via imaging, which means that we immediately have more data on its atmospheres than on the vast majority of other exoplanets. HD131399Ab is also exciting because it is one of the two lowest-mass and coolest exoplanets that has ever been directly imaged – this discovery allows us to study a planet that is not a super-Jupiter, as most of the few other directly imaged planets, but a young planet that is much closer to Jupiter in mass.

The most exciting aspect of our discovery is that we found this young planet in a triple stellar system on a wide orbit, which means it is close to a gravitational no-man’s-land. If it would be somewhat further from HD 131399A then the gravitational pull of the other two stars (B+C) would wrestle it away from A, leading to the planets eventual ejection from the system. We did not expect to find a jovian planet in such a wide orbit in a triple system – it is not clear how it could have formed there, so this was a real surprise.

RG: What are the conditions on this planet?

Apai: The planet’s orbit takes a total of 1100 Earth-years, and, for 140 Earth-years the planet is in near-constant daylight. HD131399Ab is a gas giant exoplanet. It is primarily made of hydrogen and helium – while its interior is extremely hot, the upper atmosphere we can directly observe is about 850 K (577° Celsius) in temperature. The planet does not have a surface; just like Jupiter it is a gaseous object. We expect very strong winds (hundreds of meters/second). The planet is far enough from the stars that it is not receiving too much heat from them, however it is still hot from its recent formation.

We expect that slightly below the layers we can observe the planet will have massive clouds made of silicate grains and, below that at even higher temperatures, it will have clouds made of iron droplets. In the coming months and years, we will learn more about this fascinating new world.

RG: What does this discovery tell us about what we know about solar systems?

Apai: This discovery shows that jovian exoplanets exists in orbits that are close to being unstable; our discovery shows an even larger diversity of planetary system architectures than we expected.

RG: What does your discovery tell us about our solar system in comparison?

Apai: This system is very different from ours: With three stars of different masses, two of which are in very close and short orbits, the HD131399ABC system is a much more dynamic and surprising home to planets than our Solar System is. Depending on where they are, planets in this system may see one, two, or three “suns” rising and setting in their skies -- each having a different color. Planets may have stable orbits or unstable orbits; there may be planets orbiting star A, star B, star C, or stars B+C — even orbits outside all three stars (A-B+C) are possible. So, with its complexity this system is a gift to all of us intrigued by planetary orbital dynamics.

Image credit ESO/L. Calçada