MIT researchers have found an exoplanet that orbits its star in 3.14 days (Credit: NASA Ames/JPL-Caltech/T. Pyle, Christine Daniloff, MIT)

Though it has been retired since 2018, the observations made by NASA's Kepler Space Telescope during its decade-long mission continue to allow researchers to identify new worlds in our galaxy. The latest to make headlines is an Earth-sized exoplanet that rotates around its dwarf star in just 3.14 days. The similarity to the close approximation of the mathematical constant pi — the ratio between a circle's circumference and its diameter — has earned the alien world the nickname "Pi planet."

Officially known as K2-315b, Pi planet, which lies 186 light-years from Earth, is also the 315th exoplanet — just one exoplanet away from even more fortuitous symmetry — found during the Kepler Space Telescope’s second observing mission. Called K2, the expedition began in early 2014 and monitored the sky's slivers as the spacecraft orbited around the Sun.

The team, led by Prajwal Niraula, a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS), stumbled upon K2-315b's dwarf star while combing through K2 data, dating back to 2017. After analyzing the numerous observations collected by the space telescope over several months, the scientists concluded that the periodic dips in the star’s light signaled a possible planet that was crossing in front and briefly blocking its light.

To verify their theory, the team turned to the ground-based SPECULOOS (Search for Planets EClipsing ULtra-cOOl Stars) network of 4 one-meter-long telescopes in Chile's Atacama Desert. While finding the dwarf star's location was easy enough, knowing exactly when the planet would transit across its star proved challenging. “Nailing down the best night to follow up from the ground is a little bit tricky,” says study co-author Benjamin Rackham. “Even when you see this 3.14 day signal in the K2 data, there’s an uncertainty to that, which adds up with every orbit.”

To bypass the hurdle, Rackham developed an algorithm that could forecast the best nights to witness the exoplanet zipping before its sun. The technique allowed the researchers to observe three clear transits by the exoplanet in February 2020 — two from SPECULOOS’s Southern Hemisphere telescopes and the third from the network's Artemis telescope, in the Northern Hemisphere.

The researchers were able to confirm the Pi planet's presence using the ground-based SPECULOOS telescopes (Credit: ESO / CC BY (

According to the MIT team, K2-315b moves at a blistering speed of 50 miles a second or about 180,000 miles per hour, to complete its 3.14-day orbit around its star, which they estimate is just one-fifth the size of our Sun. In comparison, Earth rotates the Sun at an average speed of 18.5 miles a second, or about 67,000 miles per hour. Though the exoplanet's mass has not been determined yet, the scientists suspect Pi planet is terrestrial like Earth. However, the tight orbit around its dwarf star causes the surface temperatures to heat up to 450 kelvins, or around 350 degrees Fahrenheit, making the exoplanet unsuitable for life. “This would be too hot to be habitable in the common understanding of the phrase,” says Niraula. Fittingly though, the temperature is perfect for baking pies.

Niraula says the connection to the numeric constant is not the only reason Pi planet is exciting. The researcher believes it may also be a promising candidate for studying exoplanet atmosphere characteristics. The MIT scientists, who published their findings in the Astronomical Journal on September 21, 2020, hope to uncover further details of the environment surrounding the Pi planet, and other exoplanets, from data collected by NASA's James Webb Space Telescope (JWST), which will be launched to space in 2021.

"There will be more interesting planets in the future, just in time for JWST, a telescope designed to probe the atmosphere of these alien worlds,” says Niraula. “With better algorithms, hopefully, one day, we can look for smaller planets, even as small as Mars.”