The Planet-Hunting TESS Discovers Its Smallest Exoplanet to Date

Thanks in large part to the Kepler Space Telescope, the number of confirmed extrasolar planets has grown exponentially in the last decade. And with next-generation missions like the Transiting Exoplanet Survey Satellite (TESS) already in orbit, more candidates and confirmed planets are being discovered all the time – many of them new and exciting ones too!

In fact, one of TESS’ most recent discoveries includes a three-planet system that orbits a star (L 98-59) located roughly 35 light-years from Earth. One of the planets, known as L 98-59b, is between the sizes of Earth and Mars – effectively making it the smallest exoplanet discovered by TESS to date. The discovery also highlights the sophistication of TESS and doubles the number of small exoplanets that are considered worthy of follow-up studies.

The paper that describes the discovery appeared in the most recent issue of The Astrophysical Journal. The international team behind the discovery included multiple NASA scientists as well as researchers from the Harvard-Smithsonian Center for Astrophysics, the Kavli Institute for Astrophysics and Space Research, and multiple universities and observatories from around the world.

Veselin Kostov, an astrophysicist at NASA’s Goddard Space Flight Center and the SETI Institute, was the lead author on the discovery paper. As he explained in a recent NASA press release:

“The discovery is a great engineering and scientific accomplishment for TESS. 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.”

As the name attests, TESS hunts for exoplanets using the method known as Transit Photometry (aka. the Transit Method). This involves observing distant stars for sudden dips in illumination, which are indications of a planet passing in front of the star (i.e. transiting) relative to the observer. By observing the extent and frequency of the dips, scientists are able to determine the existence of planets, as well as their orbital period and size.

While this method is currently the most effective way of detecting and confirming exoplanets, accounting for 3087 of the over 4000 that have been discovered so far. However, it is not very effective when it comes to spotting smaller, rocky planets like Earth. The fact that TESS was able to find not one, but three rocky planets orbiting L 98-59 is a testament to the sensitivity and capability of its instruments.

These planets (which are designated L 98-59b, c, and d) are about 0.8, 1.4 and 1.6 times the size of Earth and orbit their star very rapidly with a period of 2.25, 3.7, and 7.45 days, respectively. As Jonathan Brande, a co-author and astrophysicist at NASA Goddard and the University of Maryland, explained:

“If you have more than one planet orbiting in a system, they can gravitationally interact with each other. 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.”

While L 98-59b represents a new record for TESS, being about 10% smaller than the previous record-holder it discovered, it is not the smallest exoplanet discovered to date. That record goes to Kepler-37b, a rocky exoplanet located about 210 light-years from Earth that is only about one-third the size of Earth and 20% larger than the Moon.

However, the discovery of L 98-59b is made more impressive when you consider the fact that it orbits an M-type (red dwarf) star that is roughly one-third the size and mass of our Sun. This star is significantly less bright than Kepler-37, which is a G-type (yellow dwarf) star – similar to our Sun – even though L 98-59 is particularly bright as red dwarf goes.

Combined with the fact that it is located relatively close to our Solar System, the discovery of a three-planet system around L 98-59 makes it an appealing candidate for follow-up observations. M-type stars are the most common type in the Universe, accounting for three-quarters of the stars in the Milky Way alone.

Recent research has also found that they may be the most likely place to find rocky planets that orbit within the star’s habitable zone. Because of this, scientists are eager to learn more about the planetary systems that form around this type of stars. These include whether or not rocky planets that orbit red dwarfs would be able to retain their atmospheres given the amount of radiation they would be subject to.

Unfortunately, none of these planets orbit within the L 98-59s habitable zone. In fact, at its distance from the parent star, L 98-59b receives as much as twenty-two times the amount of radiant energy that Earth gets from the Sun. Meanwhile, L 98-59c and d receive about eleven and four times as much radiation as Earth, respectively.

However, all of these occupy the “Venus zone“, the range of distances where a planet with an Earth-like atmosphere could experience a runaway greenhouse effect, thus converting it into a Venus-like atmosphere. Based on its size, L 98-59d could be either a Venus-like world or a mini-Neptune – which a rocky core surrounded by a dense gaseous envelope.

Regardless, there is still a chance that these planets could be potentially-habitable and ongoing research will answer vital questions about these and other rocky worlds that orbit nearby red dwarfs – such as Proxima b and the seven-planet systems of TRAPPIST-1. As Joshua Schlieder, an astrophysicist at NASA Goddard and a co-author on the paper, indicated:

“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. 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.”

Fortunately, TESS will have the chance to observe the system many more times before the end of the month. At present, TESS is monitoring 24 x 96-degree regions of the southern sky (aka. sectors) for 27 days at a time. When its first year of observations are concluded this July, the L 98-59 system will have appeared in seven of the 13 sectors that make up the southern sky.

It is hoped that this will give astronomers time to refine what they know about these three confirmed planets, and perhaps to find more worlds in that system. Given that all three orbit with the star’s “Venus zone”, anything orbiting beyond would likely be in its habitable zone.

These observations will also go a long way towards establishing a catalog of rocky planets around nearby, bright stars. When the James Webb Space Telescope (JWST) takes to space in 2021, it will be using its advanced infrared imaging capabilities to gather information on these planets’ atmosphere and characterize them.

Since four of the TRAPPIST-1 worlds orbit within their star’s HZ, they are considered to be prime candidates. Kostov’s team suggests the L 98-59 planets are as well. These combined efforts will effectively bring us one step closer to determining if there are any habitable planets in our cosmic neighborhood.

Further Reading: NASA