Greenbelt, MD – A world between the sizes of Mars and Earth orbiting a bright, cool, nearby star has been discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS). 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.
“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,” Kostov stated.
A paper on the findings, led by Kostov, was published in the June 27th issue of The Astronomical Journal and is now available online.
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.
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,” Brande stated.
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.
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.