TESS Mission Discovers Smallest Planet to Date

TESS Mission Finds Its Smallest Planet Yet

Illustration of NASA’s Transiting Exoplanet Survey Satellite. Credit: NASA’s Goddard Space Flight Center

NASA’s Transiting Exoplanet Survey Satellite (TESS) has discovered a world between the sizes of Mars and Earth orbiting a bright, cool, nearby star. The planet, called L 98-59b, marks the tiniest discovered by TESS to date.

Two other worlds orbit the same star. While all three planets’ sizes are known, further study with other telescopes will be needed to determine if they have atmospheres and, if so, which gases are present. The L 98-59 worlds nearly double the number of small exoplanets — that is, planets beyond our solar system — that have the best potential for this kind of follow-up.

NASA’s TESS Mission Finds Its Smallest Planet Yet

The three planets discovered in the L98-59 system by NASA’s Transiting Exoplanet Survey Satellite (TESS) are compared to Mars and Earth in order of increasing size in this illustration. Credit: NASA’s Goddard Space Flight Center

“The discovery is a great engineering and scientific accomplishment for TESS,” said Veselin Kostov, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and the SETI Institute in Mountain View, California. “For atmospheric studies of small planets, you need short orbits around bright stars, but such planets are difficult to detect. This system has the potential for fascinating future studies.”

A paper on the findings, led by Kostov, was published in the June 27 issue of The Astronomical Journal and is now available online.

NASA’s Transiting Exoplanet Survey Satellite has confirmed the tiniest planet in its catalog so far — one of three discovered around a bright, nearby star called L 98-59. As shown in the illustrations in this video, all could occupy the “Venus zone,” the range of distances from the star where a Venus-like atmosphere is possible. The outermost planet also has the potential for a Neptune-like atmosphere. Credit: NASA’s Goddard Space Flight Center

L 98-59b is around 80% Earth’s size and about 10% smaller than the previous record holder discovered by TESS. Its host star, L 98-59, is an M dwarf about one-third the mass of the Sun and lies about 35 light-years away in the southern constellation Volans. While L 98-59b is a record for TESS, even smaller planets have been discovered in data collected by NASA’s Kepler satellite, including Kepler-37b, which is only 20% larger than the Moon.

The two other worlds in the system, L 98-59c and L 98-59d, are respectively around 1.4 and 1.6 times Earth’s size. All three were discovered by TESS using transits, periodic dips in the star’s brightness caused when each planet passes in front of it.

TESS monitors one 24-by-96-degree region of the sky, called a sector, for 27 days at a time. When the satellite finishes its first year of observations in July, the L 98-59 system will have appeared in seven of the 13 sectors that make up the southern sky. Kostov’s team hopes this will allow scientists to refine what’s known about the three confirmed planets and search for additional worlds.

“If you have more than one planet orbiting in a system, they can gravitationally interact with each other,” said Jonathan Brande, a co-author and astrophysicist at Goddard and the University of Maryland, College Park. “TESS will observe L 98-59 in enough sectors that it may be able to detect planets with orbits around 100 days. But if we get really lucky, we might see the gravitational effects of undiscovered planets on the ones we currently know.”

NASA’s newest planet hunter, the Transiting Exoplanet Survey Satellite (TESS), stares for a month at a time at sectors of the sky, watching for dips in the light from stars as planets pass in front of them, called transits. TESS will map 13 sectors each in the southern and northern sky. Credit: NASA’s Goddard Space Flight Center

M dwarfs like L 98-59 account for three-quarters of our Milky Way galaxy’s stellar population. But they are no larger than about half the Sun’s mass and are much cooler, with surface temperatures less than 70% of the Sun’s. Other examples include TRAPPIST-1, which hosts a system of seven Earth-size planets, and Proxima Centauri, our nearest stellar neighbor, which has one confirmed planet. Because these small, cool stars are so common, scientists want to learn more about the planetary systems that form around them.

L 98-59b, the innermost world, orbits every 2.25 days, staying so close to the star it receives as much as 22 times the amount of energy Earth receives from the Sun. The middle planet, L 98-59c, orbits every 3.7 days and experiences about 11 times as much radiation as Earth. L 98-59d, the farthest planet identified in the system so far, orbits every 7.5 days and is blasted with around four times the radiant energy as Earth.

None of the planets lie within the star’s “habitable zone,” the range of distances from the star where liquid water could exist on their surfaces. However, all of them occupy what scientists call the Venus zone, a range of stellar distances where a planet with an initial Earth-like atmosphere could experience a runaway greenhouse effect that transforms it into a Venus-like atmosphere. Based on its size, the third planet could be either a Venus-like rocky world or one more like Neptune, with a small, rocky core cocooned beneath a deep atmosphere.

One of TESS’s goals is to build a catalog of small, rocky planets on short orbits around very bright, nearby stars for atmospheric study by NASA’s upcoming James Webb Space Telescope. Four of the TRAPPIST-1 worlds are prime candidates, and Kostov’s team suggests the L 98-59 planets are as well.

The TESS mission feeds our desire to understand where we came from and whether we’re alone in the universe.

“If we viewed the Sun from L 98-59, transits by Earth and Venus would lead us to think the planets are almost identical, but we know they’re not,” said Joshua Schlieder, a co-author and an astrophysicist at Goddard. “We still have many questions about why Earth became habitable and Venus did not. If we can find and study similar examples around other stars, like L 98-59, we can potentially unlock some of those secrets.”

TESS is a NASA Astrophysics Explorer mission led and operated by MIT in Cambridge, Massachusetts, and managed by NASA’s Goddard Space Flight Center. Additional partners include Northrop Grumman, based in Falls Church, Virginia; NASA’s Ames Research Center in California’s Silicon Valley; the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts; MIT’s Lincoln Laboratory; and the Space Telescope Science Institute in Baltimore. More than a dozen universities, research institutes, and observatories worldwide are participants in the mission.

Reference: “The L 98-59 System: Three Transiting, Terrestrial-size Planets Orbiting a Nearby M Dwarf” by Veselin B. Kostov et al., 27 June 2019, The Astronomical Journal.
DOI: 10.3847/1538-3881/ab2459

2 Comments on "TESS Mission Discovers Smallest Planet to Date"

  1. One reason (among many) Venus is not like earth is that it has a too-strong electric wind (10 volts), which will tear away any water it had at and above the surface. It is the result of just being so much closer to the sun. This discovery is thanks to an electron spectrometer on board the European Space Agency’s spacecraft Venus Express. The high voltage drives an electric wind in Venus’s atmosphere that is powerful enough to drive all heavy ions in Venus’s ionosphere into interplanetary space. These heavy ions include oxygen ions that once belonged to water molecules. In the past, astronomers presumed that the solar wind was responsible for drying out Venus. This recent discovery shows that Venus’s electric field is the dominant factor.Earth (at 2 volts) does not have that problem. It all says a lot about some exoplanets, if, their sun is big enough. If not big enough, then other habitable zone problems emerge.
    The electric field is now considered a new and now NINTH habitable zone factor for life. Others ones include:

    liquid water (temperature)
    tropospheric ozone
    planet rotation rate
    planet rotation axis tilt
    atmospheric electric field
    Milankovitch cycles
    stellar magnetic wind
    Makes us realize how special earth is.

  2. Gabriel Feliciano | June 30, 2019 at 10:42 pm | Reply

    I would include a big moon to stabilize a planets orbit.

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