A star has been eating an orbiting planet for 85 years

A distant star called FU Orionis has been shining extraordinarily bright for the last 85 years, and it may be because it is incinerating a young, nearby planet that could be fully consumed in another 300 years

The smudge at the bottom of this simulated image is the remnant of a planet being eaten by its star
Sergei Nayakshin/Vardan Elbakyan, University of Leicester

A planet about six times the mass of Jupiter has been in the process of being ripped apart, incinerated and devoured by its star for decades. This “extreme evaporation” process has been predicted to occur and may be common in young stars, but this is the first instance of astronomers being able to observe the phenomenon.

A star called FU Orionis, which is about 1200 light years away, suddenly got hundreds of times brighter than usual about 85 years ago – and it remains that bright to this day. Sergei Nayakshin at the University of Leicester in the UK and his colleagues built a new simulation of the system and found that the brightening is probably the result of a large planet getting too close to the star and being gradually incinerated.

Past observations have shown that FU Orionis has a disc of extraordinarily hot material around it, shining around 300 times brighter than the sun. But it is far smaller and closer in than the sort of planet-forming disc we would expect to see around a young star. The disc’s temperature is in the tens of thousands of degrees.

“The disc is about 100 times smaller than the typical disc we see, but it is thousands of times brighter than most systems – it actually outshines the star,” says Nayakshin. “To be on this planet in this disc would be worse than being in hell.”

If the planet is there, as the researchers suggest the observations show, it is extremely young. It would be around 100,000 years old, and it has probably been puffed up to about the size of the sun by the intense heat, making it easier for its outer layers to be peeled off in what the researchers have called a “disc inferno”.

They calculated that it will take a maximum of about 300 years for the entire planet to be slurped up, although that will depend on whether or not it has a solid, dense core – a long-standing mystery about gas giants

While FU Orionis was the first star observed to go through this type of flare, astronomers have since discovered many others. “This process most likely happens often, and a non-negligible fraction of mass in stars may actually come from cannibalising their own planets,” says Nayakshin. A significant proportion of young planets may get devoured by their stars, he says.

Journal reference:

Monthly Notices of the Royal Astronomical SocietyDOI: 10.1093/mnras/stad1392

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