Imagine a world where a year passes more quickly than a single Earth day. That’s what’s shaking at LTT 9779b, a giant exoplanet whose existence and properties are currently inexplicable given its extreme proximity to its Sun-like star, which it orbits every 19 hours.
Located about 260 light years from Earth, LTT 9779b is the first known example of an Neptune-sized world with this type of “ultrashort” orbital period of less than a day, according to a study published on Monday in Nature Astronomy.
The strange world is also the sole member of an entirely new class of exoplanet called the “ultrahot Neptune.”
“LTT 9779b promises to shed light on the nature of Neptune-type planets, and worlds in general,” said James Jenkins, an astronomer at the University of Chile in Santiago who led the new study, in an email.
Scientists have discovered small rocky exoplanets and large Jupiter-sized exoplanets circling their stars in fewer than 24 hours. But it is unprecedented for a world like LTT 9779b to pop up in this so-called “hot Neptune desert” because this intense environment around stars is expected to blow away their atmospheres, leaving only exposed cores.
The planet was first spotted in 2018 as a weird blip in images of the LTT 9779 star system, captured by the newly launched NASA’s Transiting Exoplanet Survey Satellite (TESS). A small dip in the star’s light hinted at the possible presence of a planet that might be crossing in front of it, an event known as a transit.
Jenkins and his colleagues suspected that the ultrashort period might stem from an instrumental error, or a false positive caused by two stars in a binary system.
“When we received the information from the TESS that there was possible planet dips for this star that were happening every 19 hours and the radius could be smaller than the size of Jupiter, I was a little skeptical about what we could find,” Jenkins recalled.
“I checked through all the test outputs that were produced by the TESS team and this started to look very intriguing, something that could be quite unique,” he added.
Since most ultrashort transits end up being false positives, Jenkins and his colleagues followed up on the TESS detection with the High Accuracy Radial-velocity Planet Searcher (HARPS) instrument at the European Southern Observatory’s La Silla Observatory in Chile. HARPS can pinpoint the tiny sideways wobble of a star caused by the gravitational pull of a planet orbiting it, thereby backing up observations of transits.
To the team’s delight, the exoplanet really does exist, against all odds and assumptions.
“At this point we were all over the Moon, as this was something special, something that could provide the exoplanet community with many answers to questions we had been searching for over recent years,” Jenkins said. “We went from surprise at the beginning, to tempered excitement, to being ecstatic at the end.”
LTT 9779b appears to have somehow maintained a gassy atmospheric envelope, even in the face of the intense radiation from its star, which keeps the planet at searing temperatures of about 1,700°C (3,100°F].
“[T]heoretical modelling efforts are needed to try to understand why this planet even exists so close to its star, and still carrying an atmosphere,” Jenkins said. “These key questions on the origin of this world will give us a deeper understanding of the formation and evolutionary processes that planets undergo.”
LTT 9779 b is about five times larger than Earth and 28 times as massive, making it hulkier than Neptune but comparably dense. Its outlier status as the only ultrahot Neptune ever discovered can help scientists understand how different types of planets end up in orbital hot seats very close to their stars.
Just a few months ago, scientists announced the discovery of another planet called TOI-849b, with a size comparable to Neptune, that was also in the Neptune desert. However, TOI-849b is unusually dense and retains only a thin shell of an atmosphere, suggesting that it may be the exposed core of a much larger Jupiter-sized planet that was stripped of its skies when it wandered too close to its star.
LTT 9779 b may be undergoing an analogous process, though it appears to be a genuinely Neptune-scale world rather than the burned-out remains of an even larger planet. It’s possible that this oddball was only recently thrust into its ultrashort period, perhaps by gravitational interactions with other planets in more distant reaches of the star system. Given that the LTT 9779 system is only two billion years old, it may be that this exoplanet has a long and tortuous future in which it will ultimately be stripped of its atmosphere, similar to TOI-849b.
For now, though, LTT 9779b is the perfect place to study the evolution and behavior of ultrashort and ultrahot planets. Jenkins and his colleagues hope to learn new details about the composition of the planet’s atmosphere, or if there are any other planets in the system.
“We are already following up the planet from both the ground and space, and these observations surely promise to reveal a lot more about LTT 9779b in the near future,” Jenkins said.
With this observational data, “theoretical researchers can work to try to explain the origins of the planet,” he noted, “with the big question being: 'Why does a planet with an age of two billion years, and that orbits so close to its star that the temperature is over 1,700°Celsius, still have any atmosphere at all? Why hasn't the entire atmosphere been blown off into space long ago’?”