TESS Discovers a New Class of Exoplanet

Credit: NASA

Although exoplanetary astronomy may be a relatively young scientific field, over 4 000 exoplanets are known today. From this vast set of data, scientists have been able to determine that there are general classes which almost every exoplanet falls into, such as Super-Earth, Mini-Neptune, and Hot Jupiter. However, recent observations by NASA’s TESS (Transiting Exoplanet Survey Satellite) have identified a completely new class of exoplanet: the Ultrahot Neptune.

Astronomer James Jenkins reported the discovery of the planet in question, LTT 9779b, at the TESS Science Conference on July 29th. The planet was first identified as a candidate using TESS, and subsequent observations were conducted by HARPS (High Accuracy Radial Velocity Planet Searcher). LTT 9779b orbits the star LTT 9779, a sun-like and high metallicity star situated around 260 light years from our solar system. Thanks to these observations, astronomers were able to determine that the planet orbits its host star in a mere 19 hours, which places it extremely close to the star. Further measurements determined that the planet has a radius of 4.6 Earth radii, and a mass of 29.3 Earth masses. This positions it firmly in the Neptune-like category of planets, but it is the first such Neptune-like planet to be discovered so close to its star.

Interestingly, it appears that hardly any planets with Neptune-like mass have orbits of four days or fewer around their star. Rather, the most commonly-found planets this close to their star are Super-Earths, or planets with masses greater than that of Jupiter. These results, plus the proximity of the planet causing it to have a temperature around 2000 kelvins, categorizes LTT 9779b as a planet in the Neptunian desert. This collection of traits has never been observed before, and thus, the Ultrahot Neptune class is born.

Studying LTT 9779b will provide crucial insights into the existence of the Neptunian desert, and the evolution of gas planets. Current theories seem to suggest that gas planets often form farther out from their star, then move closer in over time. As the planet migrates nearer to its star, its orbital period decreases, and its temperature greatly increases. Furthermore, the decreasing distance between the planet and the star results in an increase in the concentration of solar wind particles, and the star slowly strips the planet of its atmosphere. Scientists hypothesize that the Neptunian desert exists because Jupiter-like planets migrate extremely close to their stars, stripping off their atmospheres, and leaving behind only a rocky core. This suggests that the newly discovered Ultrahot Neptune is perhaps a transitionary phase from Hot Jupiter to Super-Earth.

The next steps for research are to determine the rate at which LTT 9779b is losing mass due to its star. If the rate at which it loses mass is fast on astronomical time scales, then perhaps this is the reason no other Ultrahot Neptunes have been discovered until now: they simply exist for too short a time.

Read the presentation abstract here: https://tsc.mit.edu/docs/Talk_Abstracts.pdf