This distant Neptune-like planet really shouldn’t exist5 min read

A ‘Forbidden planet’ defies odds and thrives under inhospitable conditions.

It’s time for us to get acquainted with the “Forbidden Planet.” No, not the 1956 sci-fi classic—I’m talking about a new Neptune-like exoplanet found 920 light-years away, given the moniker thanks to its inexplicably ability to exist too close to its host star.

The Forbidden Planet—formally known as NGTS-4b—is the first-ever detection of a Neptunian exoplanet defying the odds and residing amongst its host star.
University of Warwick/Mark Garlick

In the world of space science, Neptune is far from a unique gem. Similar exoplanets (gas giants far bigger than Earth but exceptionally smaller than something like Jupiter) are perhaps the most common planets out there in space. But if they get too close to their host star, in an orbital zone we ominously call the “Neptunian desert,” they’re inundated with bouts of stellar radiation that evaporates their gaseous atmospheres and leaves behind a barren, shriveled up rocky core.

That’s not the case with the Forbidden Planet—formally known as NGTS-4b. It’s in the Neptunian desert, frightfully close to its host star, boasting an orbit of just 1.34 days. Yet, as researchers report in a new paper published recently in the Monthly Notices of the Royal Astronomical Society, it retains its Neptune-like atmosphere. It’s the first-ever detection of a Neptunian exoplanet defying the odds and residing cozily (relatively speaking) amongst its host star.

“As far as we are aware, it is the first exoplanet of its kind to have been found in the Neptunian Desert,” says Ed Gillen, an astronomer with the University of Cambridge and a coauthor of the new study. ”It seems as though the Neptunian desert is not completely dry, so we are now searching our data for other similar planets to help us understand whether it is greener than was once thought.”

It’s not as though the desert is barren: “we see hot and warm Jupiter-size planets, and hot and warm planets with the size of the Earth or a bit larger,” says Vincent Bourrier, an astronomer with the University of Geneva who was not involved with the study. “There are even warm Neptunes. But there are no hot Neptunes very close to their stars, hence this name of ‘Neptunian desert.’” The desert’s boundaries vary star to star, but its effects on planets is generally the same: atmospheric loss for mid-sized gas planets (unlike hot Jupiters, they don’t have enough mass to hold onto their atmosphere), turning them into bald, rocky wastelands.

The typical way scientists detect exoplanets is by watching for dips in the brightness of a star, which would happen when a planet in its orbit transits past the star from Earth’s perspective. Exoplanets like NGTS-4b—so incredibly small and so incredibly close to their stars—aren’t easily detected since the dip in brightness is so minuscule.

“Most transiting planets detected from the ground are large and hence, when they transit, they cause their stars to appear around 1 percent dimmer,” says Gillen. “NGTS-4b is much smaller, however, causing its star to appear only 0.2 percent dimmer. To detect such a small dimming from the ground is remarkable.”

Luckily, Gillen and his team had the state-of-the-art Next-Generation Transit Survey (NGTS) observing facility at their disposal. “Our telescopes are situated in the Chilean Atacama Desert, which is probably the best place on Earth to search for exoplanets,” thanks to the area’s marvellously clear skies, and absence of light pollution and radio interference, says Gillen. “We also have incredibly precise telescope machinery. Both of the aspects greatly aided us to to find this planet.”

It’s incredible to conceive of how NGTS-4b has found a way to exist so spectacularly close to its host star. A smidgen smaller than Neptune itself (okay, 20 percent smaller, but still three times the size of Earth and about 20 times more massive) it’s torched to 1,000 degrees Celsius. It’s hotter than the surface of Mercury during the day. So what’s going on here?

So far, the leading theories Gillen and his team surmise are that the planet moved into its currently neighborhood only recently, or that it once possessed a much larger atmosphere that’s actually in the process of evaporating. Its atmosphere might also just possess some strange chemistry that gives it super powers to withstand the stellar radiation.

Whatever the case, the team is eager to find more examples of gaseous Neptunian desert dwellers in order to compare situations and hone in on the reasons why an atmosphere like NGTS-4b’s could withstand such brutal conditions. The team doesn’t have much data to understand the atmosphere’s composition (the star is too faint for the transit observations to tell us anything in detail), but follow-up observations with some more powerful equipment (oh hello there, James Webb Space Telescope) might yield something useful.

The discovery doesn’t do much to move the needle in how we look for exoplanets, but as Bourrier explains, “unusual objects such as this one can tell us a lot about the processes that formed the desert,” and the potential for cosmic oases to form in these zones. And at the very least, it’s just cool to find a planet defying the odds and upending our expectations. Hot stuff, isn’t it?

Sources: • Arstechnica

Featured Image: University of Warwick/Mark Garlick

Sebastien Clarke
Sebastien Clarke

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