TESS Starts to Deliver

Early results from the world’s leading exoplanet survey.

Artist's visualization of the exoplanet LHS 3884b, a likely "lava world" newly discovered by TESS. (NASA/MIT/TESS)

NASA’s newest planet-hunting space telescope broke from the gate cleanly, discovering eight confirmed planets and more than 300 additional candidates—including about 30 that are almost ready for publication in the scientific literature—in its first few months of operations, according to mission scientists. It also recorded more than two dozen exploding stars, flares on the surfaces of red dwarf stars, and scores of other flickering astronomical objects.

“The torrent of data is starting to flow already,” said George Ricker, the principal investigator for TESS (Transiting Exoplanet Survey Satellite), during a press conference at the American Astronomical Society meeting in Seattle on January 7. “TESS is going to be a robust planet finder and a supernova machine,” added Thomas Barclay, a mission scientist at NASA’s Goddard Space Flight Center.

Launched last April, the spacecraft orbits a point in space known as L2, where the gravitational pull of Earth and the Moon are balanced. That keeps Earth out of the craft’s way and helps it maintain a steady temperature.

TESS carries four wide-field cameras. They watch a wide swath of the sky, stretching from the celestial equator to one of the poles, for about four weeks, then move to the adjacent section. The cameras photograph an entire section every 30 minutes, and snap smaller images of selected target stars every two minutes. Evidence of exoplanets can be found in both image sets.

The sky is split into 26 sections for the mission, which will take two years to complete. TESS is beginning with the skies of the Southern Hemisphere, where it has surveyed seven sections (which partially overlap), with early results released for the first four.

The early discoveries have met or exceeded expectations, Ricker said, with the cameras operating “20 percent more efficiently than we expected.”

Like the Kepler space telescope before it, TESS discovers an exoplanet when the planet “transits” its parent star, passing directly between the star and the telescope. That causes the star’s brightness to dip by a tiny amount for up to a few hours. A transit reveals a planet’s size and the length of its orbit.

Unlike Kepler, though, which studied stars up to thousands of light-years away, TESS is looking at stars within a few hundred light-years of Earth. Because the stars are close and bright, they are much easier targets for ground-based follow-up observations, which fill in a planet’s mass and other characteristics.

All of the newly discovered planets orbit quite close to their parent stars, with orbital periods ranging from a few hours to 36 days.

One of the most recent discoveries is HD 21749b, discovered in TESS’s third sector of observations. The planet is about 23 times the mass of Earth but only three times Earth’s diameter, which places it in the same range as Neptune, the outermost of the Sun’s major planets. (The mass was determined with data from TESS and archival sources, which had cataloged the star’s brightness for years.) It’s separated from its star by just 20 percent of the Earth-Sun distance, though, so its surface is too hot to be inhabited by Earth-like life.

TESS also has found evidence of a second possible planet, although it’s not yet confirmed. It appears to be about the same size as Earth but about 2.5 times as massive, making it a “super-Earth.” It orbits the star once every 7.8 days.

Other planet discoveries include LHS 3844b, an Earth-sized world orbiting a stellar pipsqueak known as a red dwarf. Although the star is a bare cosmic ember, the planet, whose mass has not yet been determined, is so close to it that “it’s probably a lava world,” said Chelsea Huang, a mission scientist at the Kavli Institute at MIT.

While TESS’s primary mission is to find exoplanets, particularly Earth-sized ones, its images are a major resource for astrophysicists as well. Because new pictures are snapped every few minutes, they reveal objects that change brightness over a short period.

The most obvious of these are the exploding stars known as supernovae. In one sector of the sky alone, TESS observed six Type Ia supernovae, which are produced by the complete disruption of white dwarfs, the collapsed cores of once-normal stars. The observations may help astronomers determine whether the explosions occurred when two white dwarfs slammed together, or when one such object “stole” so much gas from a normal companion star that it triggered a runaway nuclear blast.

TESS is even contributing new information on our own solar system, having already discovered more than 500 asteroids.

Some of the planet hunting and astrophysics work overlaps.

For example, TESS discovered a planet orbiting the star HIP 116158. It also used a technique called astroseismology to identify dozens of sets of vibrations on the star’s surface, which are caused by the motions of gas and other processes deep within the star. The vibrations reveal basic information about the star itself. It’s 1.2 times the mass of the Sun and 2.9 times its diameter; is about 4.9 billion years old, and is beginning the transition from its “normal” lifetime to its red-giant phase, when it will get much bigger and brighter.

The combination of the transit and astroseismology observations, along with follow-up work from the ground, gives scientists a detailed dossier on the planet, which is classified as a “hot Saturn”: It’s about 9.2 times Earth’s diameter but 60 times Earth’s mass, so it’s big and fluffy.

Mission scientists say TESS should be able to study thousands of supernovae and other major extragalactic events during its prime mission, with perhaps millions of variable stars, stellar flares, and other flickering objects added in. “We’re already seeing a rapid rate of discovery,” said Barclay. “It’s going to be a gold mine for people to search through.”


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