Update 15/04/2022: The study has been retracted, follow-up observations revealed that the event was a false positive. As stated by the team in the retraction: "The object might instead have been an unusually fast-moving background source, whose motion is coincidentally aligned with HD 131399, causing it to pass our common proper motion tests."
If you don’t like the night, astronomers have found the right world for you. This new discovery orbits a three-star system, guaranteeing that either all three stars are in the sky at the same time or that the planet is in between them so that when one star sets, the other two rise.
The object, called HD 131399Ab, is a gas giant about four times the mass of Jupiter. With a temperature of about 580°C (1080°F), it is one of the smallest and coldest exoplanets ever photographed. The discovery is reported in Science.
"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 co-author Daniel Apai of the University of Arizona in a statement.
The HD 131399 system is located 320 light-years away and is 16 million years old, which makes HD 131399Ab one of the youngest exoplanets found so far. The planet orbits the brightest of the three stars, with an average distance of 12 billion kilometers (7.5 billion miles).
"For about half of the planet’s orbit, which lasts 550 Earth-years, three stars are visible in the sky; the fainter two are always much closer together, and change in apparent separation from the brightest star throughout the year," said Kevin Wagner, the paper's first author and discoverer of HD 131399Ab.
The international team used the SPHERE instrument on ESO’s Very Large Telescope to discover the unusual planet. The location of this object demonstrates that massive planets may be found on wide orbits within three-star systems, although the researchers are not completely sure if the orbit is actually stable.
"If the planet was further away from the most massive star in the system, it would be kicked out of the system," explained Apai. "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."
This graphic shows the orbit of the planet in the HD 131399 system (red line) and the orbits of the stars (blue lines). The planet orbits the brightest star in the system, HD 131399A. ESO
Systems like HD 131399 are not just an astronomical curiosity, they can also help us understand how planets form. The origin of HD 131399Ab has been shaped by the severe and peculiar conditions of having three stars in such close proximity, and since it has made it this far, it may lead us to understand how more regular systems like ours formed.
"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," concluded Kevin 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."
Follow-up studies will assess how stable the system is and will look for more planetary objects orbiting stars.
This annotated composite image shows the newly discovered exoplanet HD 131399Ab in the triple-star system HD 131399. ESO/K.Wagner et al.