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An artist’s conception shows NASA’s Transiting Exoplanets Survey Satellite, or TESS, with an assortment of exoplanets. (NASA / GSFC / MIT Illustration)

Less than a year after NASA’s TESS spacecraft was launched, the scientists behind the mission have unveiled their third confirmed planet, a weird alien world that’s between Earth and Neptune in size. And hundreds of additional potential finds are in the pipeline.

The latest exoplanet on the list is HD 21749b, which orbits a star that’s about 80 percent as massive as our sun, located about 53 light-years away in the southern constellation Reticulum. Its 36-day orbital period is a record high for the TESS mission.

The “sub-Neptune” planet is about three times Earth’s size, but 23 times its mass. In comparison, Neptune is almost four times as wide as Earth but only 17 times as massive.

“This planet has a greater density than Neptune, but it isn’t rocky,” study lead author Diana Dragomir, an astronomer at MIT’s Kavli Institute for Astrophysics and Space Research, said in a NASA news release. “It could be a water planet or have some other type of substantial atmosphere.”

If it’s a water planet, HD 21749b is likely to be steamy: Based on its proximity to its parent star, the surface temperature could be in the neighborhood of 300 degrees Fahrenheit.

There might be yet another planet that’s a bit smaller than Earth and orbits even closer to the star HD 21749, making a complete round every eight days. “If confirmed, it will be the smallest planet we have found to date,” study co-author Chelsea Huang, a colleague of Dragomir’s at the MIT Kavli Institute, said today during a briefing at the American Astronomical Society’s winter meeting in Seattle.

The find is detailed in a paper submitted to Astrophysical Journal Letters.

TESS — that is, the Transiting Exoplanet Survey Satellite — is in the midst of surveying nearly the entire sky for exoplanets orbiting stars up to 300 light-years away from Earth. So far, it’s right on schedule: About a quarter of the sky has been photographed, eight months after its launch and six months after it began its two-year primary science mission.

“The spacecraft stability is really exquisite,” said George Ricker, the mission’s principal investigator at the MIT Kavli Institute.

Like NASA’s recently departed Kepler space telescope, TESS watches for the faint dip in light that occurs when an unseen planet passes in front of a star’s disk. 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.

The TESS team currently has upwards of 280 candidate planets on its list, NASA says. The first two candidates — known as Pi Mensae c and LHS 3844b — were reported in September, and they’ve now taken their place as the first two confirmed planets on TESS’ list.

Huang said several more planets have had their status confirmed, but she didn’t provide specifics. For what it’s worth, several TESS finds have been the subject of pre-print research papers.

In addition to observing exoplanets, TESS has spotted many other types of astronomical phenomena, including comets and asteroids, flare stars and mutually eclipsing binary stars, white dwarf stars and supernovae.

During the first month of data collection, TESS captured images of six supernovae, said Michael Fausnaugh, another researcher at the MIT Kavli Institute.

“Just to put that number of six supernovae in one month in context, the only other mission that could really do this was the Kepler spacecraft, and Kepler found five supernovae in four years of operation. We already have six in one month,” Fausnaugh said.

Even though the Kepler spacecraft ceased operations months ago, after nearly a decade in service, its legacy continues: Today, researchers announced that they have found a planet roughly twice as big as Earth, located within what could be its parent star’s habitable zone. The detection of planet K2-288Bb, which orbits one of the stars in a binary system located 226 light-years from Earth, took advantage of archived Kepler data as well as the sharp eyes of citizen scientists participating in the Exoplanet Explorers program.

Yet another team drew upon readings from Kepler as well as NASA’s Spitzer Space Telescope to confirm the existence of a sixth planet circling a star known as K2-138, roughly 600 light-years from Earth. The six planets appear to be in near-resonant orbits, with complex mathematical relationships between their orbital periods. That inspired the creation of a tuneful video featuring the music of the alien spheres:

If anything, TESS’ legacy could be even longer-lived than Kepler’s. The findings from TESS are meant to provide a guide for more detailed observations by space telescopes yet to be built, including the European Space Agency’s CHEOPS planet-hunting probe and NASA’s James Webb Space Telescope.

Ricker said TESS is operating so well that the spacecraft’s lifetime could be extended well beyond its two-year primary mission to overlap with the missions of those future spacecraft — perhaps for several decades.

“If you do the arithmetic, we’ve got enough propellant left for about 300 years,” he said.

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