Image showing host star Pi Mensae (big black dot), around which the new planet was discovered. The red lines show the boundary of the TESS aperture.
Image: Science and Engineering Research Council J survey/C. X. Huang et al., 2018

Say hello to Pi Mensae c—a small, Earth-like planet located nearly 60 light-years from our Solar System. It’s probably not able to sustain life, but it’ll go down in history as the first exoplanet detected by NASA’s new TESS satellite.

NASA launched the Transiting Exoplanet Survey Satellite (TESS) on April 18, 2018, and it began operations on July 25. In an early test of its powers, the satellite nabbed its first light image on August 7, which we finally got to see this past Monday.

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As its name suggests, TESS is a planet-hunting satellite. Using its four 10-centimeter optical telescopes, the satellite repeatedly scans wide fields of space, and monitors the brightness of candidate stars. This data is then analyzed by astronomers, who look for periodic dips in a star’s brightness—a possible sign of an orbiting, or transiting, exoplanet passing in front of the star. It would now appear that TESS has made its first finding of this kind.

“Here, we report on the discovery of a transiting planet around Pi Mensae, exactly the type of planet TESS was designed to detect,” write the authors in their preprint paper, which was uploaded to the arXiv server yesterday.

Where there’s a dip, there’s a planet: Light curves, or a drop in luminosity of star Pi Mensae, detected by TESS.
Image: C. X. Huang et al., 2018

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Pi Mensae, also known as HD 39091, is an unusually bright yellow subgiant (dwarf) star. This star is visible to the naked eye and is located nearly 60 light-years from Earth. Based on data gathered from July 25 to August 22, the TESS team has concluded that Pi Mensae c, as the newly discovered planet is called, is a super-Earth. It’s about 2.14 times Earth’s radius and 4.82 times Earth’s mass. Sadly, Pi Mensae c is way too close to its host star to support life. A single year on this planet lasts just 6.27 days. The planet may be uninhabitable, but its discovery is a positive sign that TESS is doing what it was built to do.

“TESS has begun to fulfill its promise to enlarge the collection of small, transiting planets orbiting bright stars,” write the authors in the study. “Such stars enable precise measurements of that planet’s mass and radius.”

This is the second planet to have been detected around Pi Mensae. Back in 2001, astronomers spotted an absolutely huge planet in this system, Pi Mensae b, which is nearly 10 times the mass of Jupiter, and the largest planet known to astronomers. This planet is so big that its surface likely glows, leading some astronomers to think it’s more a brown dwarf, a kind of failed star, than a planet. It’s also in an eccentric, or highly elliptical, orbit, taking 5.7 years to revolve around its host star.

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The preliminary survey of Pi Mensae c suggests it contains water, methane, hydrogen, and helium, in addition to a rocky, iron core. That’s pretty much all we know about this planet, but the authors of the new study say future observations made by the Gaia spacecraft and the yet-to-be-completed James Webb Space Telescope will likely reveal more about this relatively nearby exoplanet.

These observations may be far off, but astronomers won’t have to wait long for another opportunity to study this star and its planets. TESS is scheduled to collect five additional months of data from this system, allowing the team to improve its knowledge of Pi Mensae c and to search for additional transiting planets.

This is the first of what should be a slew of TESS discoveries. The mission is supposed to last for two years, during which time 500,000 stars will be studied. If all goes according to expectations, the space telescope could uncover as many as 1,000 new exoplanets.

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So that’s one down, 999 to go.

[arXiv]