Sometimes it seems as if the year just flies by. On planet Kepler 78b, it does exactly that. According to a team of scientists at MIT, the extrasolar world is so close to its sun that its year is only 8.5 hours long. That means that not only could a person go through almost three birthdays in one Earth day, but that the surface temperature would be like taking up residence in a blast furnace.
The discovery of Kepler 78b was based on curiosity by the MIT team about how close to a star a planet could orbit. “We’ve gotten used to planets having orbits of a few days,” says Josh Winn, an associate professor of physics at MIT. “But we wondered, what about a few hours? Is that even possible? And sure enough, there are some out there.”
According to the team, such planets are rare. That’s because the closer a planet is to its sun or a moon to its planet, the stronger the gravitational forces. Eventually, this reaches what is called the Roche limit. This is the distance at which a the gravity of a planet or star is so strong that an orbiting planet or moon can’t hold together and is torn apart by massive tidal forces. Even before that point, such forces can make things very unpleasant, such as on Jupiter’s moon Io, where tidal forces have turned it into a volcanic hell.
Another reason for this rarity is that planets so close to their stars face the danger of burning up or having their orbits decay and falling into their sun. They are also more likely to be small, rocky planets, which have enough tensile strength to survive the tidal pull.
Kepler 78b is one of the exoplanets discovered by the Kepler space probe, which NASA recently gave up on repairing. Kepler sought candidates by monitoring thousands of stars for dips in their light intensity that might be due to a planet passing between the unmanned space telescope and the star.
The MIT team took the Kepler data on possible extrasolar planets and supplemented it by ground observation. They found the planet by looking at readings from 150,000 stars and reducing the computational load by means of Fourier analysis, which is a way of taking complex data and reducing it to recognizable patterns.
Kepler 78b circles the star KIC 8435766 (Kepler 78), which is about 700 light years away in the constellation of Lyra. It’s a G-type star, which is the same class as the sun, but MIT states that, due to its fast rotation that indicates it hasn't had time to slow down, it’s only about 750 million years old. This is relatively young when compared to our Sun, which is over 4.5 billion years old.
Kepler 78b’s 8.5-hour year is because its distance from KIC 8435766 is only three times the star’s radius. That’s 40 times closer to its star than Mercury is to the Sun. Slightly larger than the Earth and about eight times as massive, the planet is tidally locked, which means that one side always faces the star in the same way the Moon faces the Earth, except that one side of Kepler 78b is always in daylight and the other in perpetual night.
It’s also the least likely place for a holiday imaginable. The temperature on the day side may be as high as 3,000⁰ K (2,700⁰ C, 5,000⁰ F). The night side isn't much better with a temperature of 2,700⁰ K (2,400⁰ C, 4,400⁰ F). This means that the planet’s surface is one gigantic ocean of boiling lava and the atmosphere, long blasted away by solar winds, would consist of traces of heavier elements.
“You’d have to really stretch your imagination to imagine living on a lava world,” Winn says. “We certainly wouldn't survive there.”
One side benefit of this Dantean heat is that it’s so great that the scientists could detect light emitted by the planet itself. This incandescence raises the possibility of learning about the composition of the planet using larger telescopes by means of spectral analysis. It might even be possible to determine its mass, which would be a first for an exoplanet.
If a planet with an 8.5-hour day isn’t strange enough, the team has found one even shorter. Called KOI 1843.03, it may have a year only 4.2 hours long, but according to a paper published in Astrophysical Journal Letters, it hasn't been properly vetted yet. If confirmed, the planet will probably prove to be made of solid iron, otherwise tidal forces would have destroyed it.
“Just the fact that it’s able to survive there implies that it’s very dense,” says Winn. “Whether nature actually makes planets that are dense enough to survive even closer in, that’s an open question, and would be even more amazing.”
Winn says that despite being so close to a star, it's possible for some sort of life to exist on a planet with a year of very short duration, provided the star is cool enough. “If you’re around one of those brown dwarfs, then you can get as close in as just a few days. It would still be habitable, at the right temperature.”
The MIT findings were published in the Astrophysical Journal.