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(NASA Ames/JPL-Caltech/T. Pyle, Christine Daniloff, MIT)

Astronomers Discover 'Pi Earth' Exoplanet Orbits Its Star Once Every 3.14 Days

22 SEPTEMBER 2020

Every exoplanet is special in its own way, but a newly discovered exoplanet 186 light-years away is an especially delicious treat. It's a smallish world around the same size as Earth, whipping around its star on an orbit that takes just 3.14 days.

 

That's extremely close to the mathematical constant π (Pi), the number that describes the ratio of a circle's circumference to its diameter. So, even though the exoplanet's official name is K2-315b, its discoverers have nicknamed it "π Earth."

The discovery of the star has been several years in the baking. The first hints of its existence emerged back in 2017, when the Kepler Space Telescope (RIP) was conducting its second, extended mission.

In the light of a small, dim red dwarf star just 20 percent the size of the Sun, Kepler detected 20 tiny dips at regular intervals. This is the main way we search for exoplanets - such dips often mean that a planet is crossing between us and the star in question, something known as a transit. Those transits register as very slight fluctuations in starlight.

But just one collection of dips isn't enough to confirm an exoplanet detection, so, armed with the Kepler data, astronomer Prajwal Niraula and his colleagues set out earlier this year to study the star with SPECULOOS.

This is a network of telescopes working together to specifically search for Earth-sized exoplanets around dim dwarf stars.

 

On three observing runs in February, March, and May, the team turned SPECULOOS telescopes in the direction of the star K2-315. They observed three more dips, and obtained a spectrum of the star using the HIRES instrument at the W. M. Keck Observatory.

The dips were completely consistent with the 3.14-day timing recorded by Kepler three years ago. And the spectrum of the star confirmed that the dips are indeed the result of a transiting exoplanet.

We can tell a lot from stellar transits. Just the amount of light the star dims by can tell us how big the exoplanet is. From this, Niraula and his colleagues determined that K2-315b is around 95 percent of the size of Earth.

At that size, the planet is probably rocky, like Earth, Venus, Mercury, and Mars, but other compositions can't entirely be ruled out.

For that, we need Doppler spectroscopy. Even a very small planet tugs a little bit on its star as the pair move around a mutual centre of gravity, and those movements generate very small changes in the wavelengths of light from the star - compressed as the star moves towards the viewer, and stretched as it moves away.

 

Exactly how much the star moves can provide a mass for the exoplanet. Fluffy gas giants are low density, so if you have a large planet with a relatively low mass, you can infer that it's gaseous. Rocky planets are more dense, so their mass-to-size ratio is higher.

Even if K2-315b is rocky, however, there's no chance of habitability to life as we know it.

With an orbit of just 3.14 days, even at its breakneck orbital speed of 81 kilometres per second (50 miles per second)(Earth's orbital speed is 29.78 kilometres per second, or 18.5 miles per second), it's so close to its star that its surface would be hellaciously hot. It would heat up to around 450 Kelvin (177 degrees Celsius, or 350 degrees Fahrenheit).

However, the planet could be a good target for follow-up study. That's because small, dim stars don't glare as brightly as their larger siblings, which means they present a better opportunity to look for atmospheres when the exoplanet transits.

As the light of the star passes through an exoplanet atmosphere, the light should change slightly as different elements absorb or emit certain wavelengths.

Even the atmosphere of an uninhabitable exoplanet would be a fascinating thing to study, and the team estimates that the upcoming James Webb Space Telescope would need just 40 transits to have enough data to make a detection.

That's for future science, though. In the meantime, the team plans to continue using SPECULOOS to try to find more targets.

"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."

The research has been published in The Astronomical Journal.