The TESS spacecraft found a blip in starlight that revealed an exoplanet NASA's Goddard Space Flight Center

An exoplanet about twice the size of Earth has been found orbiting a star called Pi Mensae about 60 light years away. It’s the first world discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS) – a spacecraft that began surveying the galaxy just two months ago.

“This is one of the first objects we looked at,” says Chelsea Huang, a TESS scientist at the Massachusetts Institute of Technology. “We were immediately saying ‘hey this is too good to be true!’”

The alien world, Pi Mensae c, takes 6.27 days to complete an orbit around its parent star, Pi Mensae. That star is so bright, it’s visible to the naked-eye from a dark-sky site in the southern hemisphere. It was previously found to have a planet the mass of ten Jupiters circling it.


TESS works by watching thousands of nearby stars in the Milky Way, looking for signs of planets transiting them – miniscule dips in the stars’ brightness can give away the presence of an orbiting planet momentarily passing in front of them. Such a blip in the light from Pi Mensae was identified in the first month of observations beamed home by TESS, says Huang.

The team confirmed the presence of Pi Mensae c by examining separate studies of the light of its parent star made by ground-based observatories. This analysis revealed a tiny tell-tale wobble in the star’s motion that the researchers attribute to the gravitational pull of TESS’s newly detected world.

Based on the planet’s radius it’s likely a mini-Neptune with a gaseous hydrogen and helium atmosphere, says Hugh Osborn, an exoplanet researcher at the Marseille Astrophysics Laboratory in France. “Pretty much all the planets we find at that radius have this gassy layer,” he says. “So it wouldn’t look anything like Earth.”

Shortly after the first detection, NASA announced that TESS had found a second planet. This new world, called LHS 3844 b, orbits a small star 49 light years away. It is smaller than Pi Mensae c, only 1.3 times the size of Earth, but much hotter at an average temperature of about 532°C.

It is so close to its star that it orbits once every 11 minutes. The TESS team say that it may be the easiest planet to observe that is less than twice Earth’s size. That means it will be easier to check if it has an atmosphere, although it is so hot that it likely does not.

Additional reporting by Leah Crane

Reference: arxiv.org/abs/1809.05967, arxiv.org/abs/1809.07242v1

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