Astronomers Find ‘Pi Planet’ With 3.14-Day Orbit


The longer we study the universe, the more exoplanets we find. Many of these discoveries are notable because of how Earth-like they are or because of the number of planets crammed into a single solar system. The rocky planet K2-315b, on the other hand, is notable because of its orbital period. It takes 3.14 Earth days to complete an orbit of its star. Astronomers have therefore dubbed it “pi planet.”

The planet’s name gives a hint of its origins. This is the 315th exoplanet discovered in the data from the Kepler K2 mission. That was the second phase of Kepler’s life after several of its components failed, limiting its ability to remain pointed in any one direction. That was a problem for its planet-hunting activities, but NASA managed to partially revive it by using the solar wind to stabilize Kepler along several parts of its orbit. 

Kepler used the transit method to find planets, which requires scanning distant stars for long periods of time to monitor for dips in light. Those dips can signal a planet has passed in front of the star (known as EPIC 249631677). Therefore, the transmit method is best at detecting larger planets that orbit close to the star. Even though Kepler shut down some time ago, teams like the one at MIT are still poring over its data in search of new planets like K2-315b. 

This new world is closer to its star than Mercury is to the sun, but it’s otherwise potentially Earth-like. The upshot, of course, is that its year is just 3.14 Earth days. Yes, it’s an arbitrary human-centric measurement, but it’s still fun. 

How to use sunlight (photon pressure) as Kepler's third reaction wheel

Astronomers estimate K2-315b to be 0.95 Earth radii, but the team hasn’t determined the mass yet. Regardless, it’s not looking like a pleasant environment. Because K2-315b is so close to the star, it has a surface temperature of about 350 degrees Fahrenheit (176 degrees Celsius). As MIT points out, that’s hot enough to bake real pies. 

Data from the K2 mission is often not enough to confirm a planet on its own. The MIT researchers used the SPECULOOS telescope array, which consists of five 1-meter telescopes (four in Chile and one on the largest of the Canary Islands). After nailing down a time when they were likely to catch a transit, the team pointed the array at EPIC 249631677. Sure enough, they spotted the pi planet with its coincidental orbit. 

The star is about 185 light-years away, which isn’t far in the grand scheme. Future instruments like the James Webb Space Telescope might be able to get a better look at this rocky mathematical happenstance.

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