Doomed egg-shaped exoplanet is death-spiraling into its star
Astronomers have discovered that a distant, scorching hot planet twice the size of Jupiter is on a death-spiral trajectory that will send it careening into its parent star. A crash is expected to happen relatively soon, cosmically speaking.
Researchers have predicted for some time that this planet, named WASP-12b, will eventually take a dive into its star that's located around 1,400 light years from Earth. However, these new findings have shortened the time WASP-12b has left.
Previous estimates gave WASP-12b around 10 million years before its inevitable demise, but these researchers say its more likely the planet smashes into its star way sooner.
"According to our calculations, the planet will crash into the star [WASP-12] in just 3 million years, an incredibly short amount of time considering the star only appears to be 3 billion years old," Pietro Leonardi, research lead author and University of Padova scientist, told Space.com.
In other words, this may seem like an incredibly long time, but the fact that stars like the sun live for around 10 billion years means it is a very (very) short period on cosmic scales.
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WASP-12b gets too close for comfort
In general, the doomed planet WASP-12b orbits its yellow dwarf star so closely that it almost fits an entire year into a single Earth day. This proximity classifies WASP-12b as an "ultra-hot Jupiter" planet, a name that is fitting considering radiation from this star ceaselessly belts the planet, giving it a surface temperature of around 4,000 degrees Fahrenheit (2,210 degrees Celsius).
That isn't the only thing, however, that makes this doomed world an extreme exoplanet unlike anything found in the solar system. The immense gravity felt by WASP-12b at just 2.1 million miles from its star generates such great tidal forces that it is now shaped like an egg.
This gravitational influence also strips material from WASP-12b, which forms a disk of matter around the planet's yellow star.
When it was discovered in 2008, WASP-12b was the hottest planet ever seen, a record it surrendered in 2018 to another world dubbed Kelt-9b. At the time, WASP-12b was also the closest planet to its star, though that record is now held by K2-137b, which sits just over half a million miles from its red dwarf star that's located some 322 light-years away from Earth.
Though WASP-12b is just one of many hot Jupiter exoplanets discovered since the mid-1990s, something has always set this planet apart.
WASP-12b appeared to experience variations in the time it takes to orbit its star, for instance. Previous theories had attributed this to factors such as the planet's position with regard to Earth and a gradual shift in orbit.
Leonardi and colleagues investigated the timing variation of WASP-12b by looking at 28 observations of the planet taken as it crossed, or "transited," the face of its parent star. This was done in collaboration with the Asiago Search for Transit Timing Variations of Exoplanets (TASTE) project. These observations were collected over a period of 12 years between 2010 and 2022 by the Asiago Observatory in Italy.
Not only did this study reveal that WASP-12b's fiery fate in around 3 million years is a result of a phenomenon called "tidal dissipation," but it also gave the team the first indications that the planet's yellow star is very active. During periods of high activity, stars are covered by more dark patches called sunspots and experience more extreme outbursts of charged particles in the form of plasma. This means that the team may have caught WASP-12b as it experienced an even more violent blasting by its star than usual.
One surprise that the team's analysis delivered was some evidence that suggests the dwarf star had already come to the end of its main-sequence lifetime, a period that sees stars burning hydrogen in their cores.
For low to intermediate-mass stars like WASP-12, which has a mass and width around 1.5 times the mass and width of the sun, the end of core-hydrogen burning triggers a period of life called the "sub-giant phase," during which hydrogen burning moves to the star's outer layers.
"According to the tidal theory, the dissipation we see in the system is too strong to be explained by a main sequence star. If the star had already left the main sequence and entered its sub-giant phase, this could be easily explained," Leonardi said. "To test this theory, we used high-resolution optical spectra from the High Accuracy Radial Velocity Planet Searcher in the Northern Hemisphere (HARPS-N) to derive the stellar parameters of the star and infer its evolutionary stage.
"However, according to our results, the star is still in the main sequence and has not yet entered its sub-giant stage."
This means the team still needs to explain how rapid tidal dissipation can be caused by a main sequence dwarf star.
In around 3 million years, when WASP-12b finally plunges into its star, this will trigger changes that observers should be able to see from Earth — assuming there is still intelligent life left on our planet.
"When the planet inevitably crashes into the star, the first indication will be an outburst of luminosity, which will see the star become hundreds of times brighter than it is today," Leonardi said. "This increase will not last long and will quickly fade away. But maybe the humans of the future could be there to see it and study it."
The team's research is currently available on the paper repository arXiv.
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Leonardi thinks that the findings regarding the doomed status of WASP-12b could indicate that other ultra-hot Jupiters could also be on collision courses with their stars.
"We still have to figure out if what we observed is a unique scenario or a common event in the universe," Leonardi said. "According to some population studies, the number of hot Jupiters orbiting very close to their stars decreases when we observe older stars, so this could be an indication that many planets experience tidal decay and crash into their stars."
Leonardi added that he is now working with the team behind the European Space Agency (ESA) mission CHaracterising ExOPlanet Satellite (CHEOPS) to determine the orbital decay rate of other hot Jupiters.
"This study is just the beginning of a long search for orbital decay," he concluded.
The team's research is published on the paper repository arXiv.
