The new planet, K2-288Bb, orbits the smaller, dimmer star every 31.3 days.
The researchers also detected hints of another, smaller planet in the system, a planet that would have an orbital period of 7.8 days.
"It's the coolest small planet that we know of around a star this bright", Dragomir said.
"We learned from Kepler that there are more planets than stars in our sky, and now TESS will open our eyes to the variety of planets around some of the closest stars", Paul Hertz, the director of NASA's astrophysics division, said in March 2018.
"It's a very exciting discovery due to how it was found, its temperate orbit and because planets of this size seem to be relatively uncommon".
When a planet passes in front of a star as viewed from Earth, the event is called a "transit".
TESS - that is, the Transiting Exoplanet Survey Satellite - is in the midst of surveying almost the entire sky for exoplanets orbiting stars up to 300 light-years away from Earth.
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Those first three days of data were ignored, and errors were corrected in the rest of the data gathered. For their new analysis, the researchers looked through this data, collected between July 25 and October 14. Possible planets can be spotted by studying dips in light when the planet moves before its star. As the satellite only collects data from a sector for 27 days, it's hard to identify planets with orbits longer than that time period; by the time a planet passes around again, the satellite may have shifted to view another slice of the sky. The exoplanet lies in the stellar system K2-288, which has two dim, cool stars.
Tess will spend about two years surveying 200,000 of the brightest stars near the sun to search for planets outside our solar system. "But we had this one transit, and knew something was there".
Volunteers re-examined the observation data, which is when they came across the new planet. In May 2017, volunteers noticed the third transit and began an excited discussion about what was then thought to be an Earth-sized candidate in the system, which caught the attention of Feinstein and her colleagues.
"Because there was an interruption in data around that time, we initially didn't see a second transit, and were pretty disappointed", Dragomir recalls. "TESS found as many in its first month".
She and her colleagues compared the pattern to the first full transit they had originally discovered, and found a near ideal match - an indication that the planet passed again in front of its star, in a 36-day orbit.
"We've confirmed three planets so far, and there are so many more that are just waiting for telescope and people time to be confirmed", Dragomir said.
Even if TESS records the looked-for pattern of dimming and brightening, astronomers have to make ground-based observations to confirm that what they're seeing is truly an exoplanet rather than some other type of phenomenon. "So it's going really well, and TESS is already helping us to learn about the diversity of these small planets".
"Reorienting Kepler relative to the Sun caused miniscule changes in the shape of the telescope and the temperature of the electronics, which inevitably affected Kepler's sensitive measurements in the first days of each campaign", said study co-author Geert Barentsen, an astrophysicist at NASA's Ames Research Center, in a statement.