Just as astronomers were basking in the afterglow of the TRAPPIST-1 discovery, an unexpected treasure trove of eight planetary systems with more than twice as many alien worlds have emerged from the darkness of space, and some of them are mind-blowingly bizarre.
With its Echelle Spectrograph Instrument, ESO’s HARPS (High Accuracy Radial velocity Planet Searcher) recently found everything from small worlds with even smaller orbital periods to super-Earths and Neptune-mass spheres around stars as bright as the sun. The discoveries are so new that they are still pending tests to determine that all of them really are planets, and not just false positives picked up from random planet and star-induced signals. Among them are several systems with double Neptune-mass planets and one with three of them orbiting in the habitable zone, but there is another whose rarity makes it even more fascinating.
Planets are not often found in binary systems, but two super-Earths and two of Neptunian mass orbit the star HD 201781, while a world the size of Jupiter orbits the brighter HD 20782. Their orbits range from about the length of a work week here on Earth to an eccentric 595 days for the Jupiter-sized sphere. We hardly know anything about this low-mass giant except for its mass and orbital period. Eccentric planets, such as this enormous low-mass orb, intrigue scientists because none exist in our solar system. That they even exist at all could be evidence that their formation and evolution history hides some strange phenomenon.
“These planets are really interesting because they have these longer orbital periods, and they complement the planets from transit surveys like Kepler,” says astronomer Jason Dittman of Harvard University. “If we want a complete picture of what sort of planets exist in the universe, then discoveries like these are vital to the story."
Highly eccentric planets are believed to form from circular discs, whose shape means they must have been thrown into such an uncommon orbit by a star or another planet. How gravitational tides from the dual stars in a binary system affect the formation of planets such as HD 20782’s is nebulous. Whether tidal disruption perturbs such planets or ongoing star-planet interactions result in planetary instability is still being investigated. The question that hangs in space is whether a modeling experiment could reveal whether HD 201781 or its orbiting planets could, for either of those two reasons, be the culprits behind the HD 20782 planet’s higher eccentricity.
“Detecting planets with periods that are several years long takes a long time to do because you want to see the planet go around the star a couple of times,” acknowledged Dittman. Meaning, it’s going to take time to collect enough data on the Jupiter-size planet’s eccentricity to get to that elusive eureka moment.
No word on aliens yet, but with at least three potentially habitable planets between these star systems, theories are bound to spawn.