It’s no secret that our galaxy is stuffed with alien worlds — but that doesn’t mean there aren’t surprises hiding in the dark.
Take, for example, the exoplanet du jour: a recently unveiled, massive planet orbiting an itty-bitty star. Called NGTS-1b, the planet is a beast about the mass of Jupiter. But it orbits an old, dim red dwarf just half the width of the sun. It’s the largest planet relative to its star found to date, and is tucked so close to that star that a year on NGTS-1b lasts just 2.6 Earth-days.
It’s an arrangement that scientists didn’t expect to see, because current theories of planet formation suggest that small stars grow small planets, and bigger stars grow bigger planets.
Spotting a Giant
Spotted by the exoplanet-hunting Next-Generation Transit Survey’s telescopes in Chile, the system is about 600 light-years away and is likely “very old,” scientists report in a study set to appear in the Monthly Notices of the Royal Astronomical Society.
Astronomers found the giant world by watching for brief blips in starlight that occur as the planet crosses its star’s face. Called transits, these short dimming events have been responsible for revealing the presence of thousands of exoplanets in the Milky Way galaxy, most of which were seen by NASA’s Kepler spacecraft.
But Kepler didn’t tell anyone much about systems like NGTS-1b.
Of the worlds it spotted, precious few were giant planets in snuggled-up orbits — although at least one of those, reported by Harvard University’s John Johnson, contains a similarly large planet orbiting a tiny star. “I wouldn't call this [new planet] an oddball, in that it wasn't unexpected,” Johnson says. “But it certainly is a rarity in our neck of the galaxy, and as such I, as an exoplanetary scientist, find this interesting.”
Scientists are still trying to figure out whether those planets formed where they now live, or if they grew up farther out and migrated inward. And red dwarf stars like the new planet’s sun, though they are plentiful, are hard to study because they’re generally so dim.
“NGTS-1b was difficult to find, despite being a monster of a planet, because its parent star is small and faint,” says the University of Warwick’s Peter Wheatley in a statement. “Small stars are actually the most common in the universe, so it is possible that there are many of these giant planets waiting to found.”
Until now, Wheatley and his colleagues say, scientists knew of only two similarly large planets orbiting these tiny hosts, called M dwarf stars, and neither of those systems holds a planet as massive as NGTS-1b.
“Hot Jupiters around M dwarfs are super-rare,” says Lauren Weiss of the Université de Montréal.
A Planetary Mystery
Another puzzle: Such large planets shouldn’t really be there, according to current theories of planet formation. In general, scientists suspect that larger stars harbour larger planets. Simply put, this is because big stars and planets emerge from a large cluster of starting materials such as gas and dust. Smaller stars, like red dwarfs, grow from smaller amounts of ingredients, so smaller worlds form around smaller stars.
The new planetary system breaks that idea, though it’s not clear whether it’s time to completely rewrite the story of planet formation, or just add an addendum.
Luckily, the newly found giant planet is near enough that the James Webb Space Telescope should be able to study its atmosphere, Weiss says. And that, in turn, should help point toward where the planet grew up relative to the star.
“We still have no clue how hot Jupiters form around sun-like stars, and it’s even harder to figure out how they form around M dwarfs,” Weiss says.