For the first time ever, scientists have glimpsed the exposed core of a giant planet, providing a rare opportunity to peer at the heart of an alien world.
The planet, called TOI-849b, is slightly smaller in size than Neptune, but up to three times more massive, making it “the densest Neptune-sized planet discovered so far,” according to a study published on Wednesday in Nature. The planet was originally discovered in 2018 by NASA’s Transiting Exoplanet Survey Satellite (TESS), but it wasn’t until last year that scientists began to grasp its bizarre properties, which have never been observed in any star system before.
“It’s incredible to be able to make these kinds of discoveries, and more so now we can let people know about it,” said David Armstrong, an exoplanet researcher at the University of Warwick and lead author of the new study, in an email.
“When the data started coming in for this one, we actually thought it was going to be an error,” he added. “The unusually high mass in the first few datapoints led us to believe it was a false positive signal, but when we carried on observing the star to be sure we realised we were looking at something special.”
The bizarre world, which is about 730 light years from Earth, orbits extremely close to its Sun-like star, resulting in a year of just 18 hours. As a result of its proximity to its star, the planet’s surface likely experiences searing temperatures of roughly 1,500°C (2,700°F).
TESS spotted TOI-849b by capturing the slight dip in the light of its star as the planet passed in front of it, which is known as the transit method of exoplanet detection. The discovery quickly attracted attention from scientists because TOI-849b is a Neptune-sized planet that occupies what’s known as the “Neptunian desert.” This term refers to a zone located close to stars where planets that are Neptune’s size, or larger, are extremely rare because their host stars can blow away their atmospheres.
To get a better look at TOI-849b, Armstrong and his colleagues spent the latter half of 2019 examining the planet with specialized instruments, such as the High Accuracy Radial Velocity Planet Searcher (HARPS) at La Silla Observatory in Chile. Those observations revealed that TOI-849b was much more massive than would be expected for a planet of its size.
There are two likely explanations for this odd finding: Either the planet is the naked core of what was once a Jupiter-scale gas giant, or it is a “failed gas giant” that never accumulated a gassy envelope in the first place. While there is probably a thin shell of an atmosphere surrounding the planet, perhaps containing water vapor, it is clearly nothing like the deep and layered cloud cover that envelops our solar system’s gas giants, such as Jupiter and Saturn.
The cores of planets like Earth are typically buried under hundreds of miles of crust and mantle, but the cores of Jupiter-sized gas giants are maelstroms of dense heavy elements surrounded by tens of thousands of miles of gas.
If TOI-849b was once a “hot Jupiter,” a common class of sweltering Jupiter-sized worlds with tight orbits, it may have been stripped of its atmosphere by pulling an Icarus and getting a little too close to its star.
“Hot Jupiters slowly migrate towards their host stars,” Armstrong explained. “When they cross a certain distance, known as the Roche limit, the tidal forces between the star and the planet grow so great that the planet is ripped apart, which is what’s called tidal disruption.”
Such a disruption could have robbed TOI-849b of any nascent atmosphere it once had. It’s also possible that the planet lost its gas giant status due to “thermalization events,” according to the study.
“The thermalization events we hypothesised are similar, but occur when a planet is on a very elliptical orbit,” Armstrong said. “It passes very close to the host star at one point in its orbit, and builds up a lot of energy in the planet’s atmosphere at that point.”
‘Over time, that energy can be so great that the atmosphere is blown away,” he continued. “One way for a planet to get on such a highly elliptical orbit is the influence of other planets in the system.”
In other words, TOI-849b’s interactions with both its host star and its sibling planets could account for the absence of its atmospheric sheath.
However, it’s also possible that this weird world has simply always been an exposed core. If the planet formed in a region of the star system that was low on gas and dust, or if it was born later in the lifespan of the system when much of the gas had already been collected by other planets, it may never have evolved into a gas giant at all.
Future observations of the system may help to resolve the mystery of the planet’s origins, and even probe any gases in what’s left of its atmosphere.
“We are trying to measure the alignment of the planet’s orbit with the spin of its host star, which will tell us more about the past history of TOI-849b,” Armstrong said. “In the longer term we would like to measure its atmospheric constituents, but that will take next-generation telescopes to achieve.”
These follow-up studies may also constrain the composition of this exposed core, offering scientists an unprecedented look at a planet’s interior, from hundreds of light years away.