A distant Earth-sized planet is orbiting so close to its parent star that gravity is stretching it to an extreme degree, turning it egg-shaped.
A handful of known exoplanets are extremely close to their parent stars, which exposes them to incredibly harsh conditions. These “ultra-short period” planets – they take less than a day to complete one loop around their stars – are blasted with radiation and often have surfaces composed entirely of lava.
Now, at the University of Hawaiʻi and his colleagues have discovered one of the most extreme examples of these planets. A year on TOI-6255b goes by in just under 6 hours, and the intense gravitational force from its star has warped the planet’s shape to be 10 per cent longer in one direction. “The system is the most highly distorted that we know so far,” says Dai.
Advertisement
The exoplanet is remarkably similar to Earth in some ways, with a radius just 1.08 times that of our planet and a mass 1.44 times as great. It is also likely to have a similar composition of iron and silicon.
But the powerful radiation from the host star has heated one side of the planet to at least 1100°C (2012°F). Standing on its surface, you would look down at an ocean of molten rock beneath you and up at a star that appears 80 times larger than the sun does in Earth’s sky. “You can imagine, if you’re on this hellish planet, life wouldn’t be very pleasant,” says Dai.
Dai and his team discovered TOI-6255b using a relatively new planet-hunting telescope at the W. M. Keck Observatory in Hawaii. The Keck Planet Finder, which started looking for planets two years ago, can record the brightness and motion of stars with a much higher degree of precision than previous tools. Dai and his team used the instrument to infer the planet’s properties, like its mass and radius, in great detail.
“Very often you don’t get the wealth of data that the authors have gotten for a system like this,” says at Boise State University in Idaho. “Having all of that for this really gives us a deep insight into these kinds of systems that it’s very hard to get.”
We are probably observing the planet at the very end of its life, says Dai. Over the next few hundred million years, it will become even more stretched until it slowly disintegrates and is eventually eaten by its host star. While this is a relatively common occurrence – scientists recently estimated that one in 12 stars cannibalises its own planet – it is much rarer to see the final stages of this process. “The planet probably started out with much longer orbital periods, so we are catching it right at the end,” says Dai.
The egg-shaped exoplanet also provides an opportunity to observe powerful magnetic fields in action. In our own solar system, planets are too far away to feel the effects of the sun’s magnetic field, but TOI-6255b could be bathed in its star’s magnetic field. “The planet probably sweeps through these loops of magnetised plasma because it’s so close,” says Jackson. While Dai and his team didn’t look for these effects in their study, they should be able to study it in future work, says Jackson.
arXiv
