TESS Finds a Super-Earth and two Mini-Neptunes in a Single System

The field of extrasolar planet studies continues to grow by leaps and bounds. Currently, 5,090 exoplanets have been confirmed in 3,816 systems, and another 8,933 candidates are awaiting confirmation. The majority of these have been Neptune-like gas giants (1,779), gas giants comparable to Jupiter or Saturn (1,536), and rocky planets many times the size of Earth (1,582). The most effective means for finding exoplanets has been the Transit Method (aka. Transit Photometry), where periodic dips in a star’s brightness are seen as an indication of a planet passing in front of its star (transiting) relative to the observer.

Using data from NASA’s Transiting Exoplanet Survey Satellite (TESS), an international team of astronomers has discovered a three-planet system orbiting a Sun-like star (HD 22946, or TOI 11) located about 205.5 light-years. Based on size estimates yielded from their transits, the team theorizes that these exoplanets consist of a rocky planet several times the size of Earth (a Super-Earth) and two gas giants smaller than Neptune. Given its proximity, this system could be ideal for follow-up studies and characterization with the James Webb Space Telescope (JWST).

The international team consisted of 55 astronomers from the European Southern Observatory (ESO), the National Institute for Astrophysics (INAF), the National Institute of Nuclear Physics (INFN), the Institute of Space Astrophysics and Planetology (INAF-IAPS), the NASA Exoplanets and Stellar Astrophysics Laboratory, the NASA Exoplanet Science Institute (NExScI), the SETI Institute, the Harvard & Smithsonian Center for Astrophysics (CfA), the Perth Exoplanet Survey Telescope (PEST), Curtin Institute of Radio Astronomy (CIRA), and many universities.

An artist’s illustration of the mini-Neptune TOI 560.01 losing its atmosphere and transitioning to a super-Earth. Credit: Adam Makarenko (Keck Observatory)

The research was led by Luca Cacciapuoti, a Ph.D. student at the University of Naples Federico II and the ESO, and the paper that describes their findings recently appeared online. As they indicate in their paper, the two mini Neptunes – designated TOI 411.01 and 411.02 (planets c and b) – were identified using TESS data and were found to have orbital periods of 9.57 and 4.04 days, respectively. They also detected a single transit-like signal that indicated the presence of a third planet (the Super-Earth) with a period of about 46 days. As Cacciupoti explained to Universe Today via email:

“In the case of TOI-411, two planets were found by this first quick automated search. At the beginning of 2021, though, I was leading another project related to TESS, Planet Patrol. This is a citizen science project (run on Zooniverse first, and then privately) with which we analyzed TESS candidates to look for false positives (imposter signals similar to planets generated by different sources).”

As Cacciupoti and his Planet Patrol indicated in a previous study (titled “The TESS Triple 9 Catalog: 999 uniformly vetted exoplanet candidates“), they also noticed a new signal in the lightcurve of TOI-411 that was a possible indication of a third planet. Ordinarily, astronomers would attempt to confirm a possible transit signal using the Radial Velocity method, where spectra is obtained from the parent star to determine its velocity back and forth (relative to the observer). This is then used to calculate the gravitational forces acting on the star, which may result from a planetary system. Said Cacciupoti:

“What we did in the paper, instead, it’s been making the best possible use of TESS data as well as other ground-based observations! We didn’t use high-resolution spectra of the star but rather eliminated [with] high confidence any other possible scenario that could cause the signal. Hence, we excluded that the signal might be caused by stellar binaries, stellar spots, and other false positive scenarios. We used Tess data, high contrast imaging, and ground-based photometry.”

Artist’s impression of a Super-Earth orbiting the Sun-like star HD 85512 in the southern constellation of Vela (The Sail). Credit: ESO

Cacciupoti and his team then sought to determine the stability of the planetary system using N-body simulations. This consisted of computer simulations that tracked the motion of the two gas giants around their star for millions of orbits to see if the planets would survive or if any instability would eventually occur. They then consulted the scientific literature on the dynamics of planetary systems to assess whether pairs of planets could host a third in between them.

“This criterion allows us to tell if there might be space enough for a fourth planet between planets c and d,” Cacciupoti said. “This wouldn’t be the first time of such an occurrence, with several examples in literature in which new planets were found with different techniques that eclipses couldn’t find (usually because of the inclination of the orbit or its long period).”

Cacciupoti and his colleagues hope to conduct follow-up observations of this system using the CHaracterising ExOPlanet Satellite (CHEOPS) and the High Accuracy Radial velocity Planet Searcher (HARPS) on the ESO’s 3.6-meter telescope at the La Silla Observatory. Whereas CHEOPS searches for exoplanets around bright stars using the Transit Method, the HARPS spectrograph conducts Radial Velocity measurements to determine the presence of exoplanets. They also hope to take advantage of the James Webb Space Telescope (JWST) and its advanced infrared optics and spectrometers.

Already, Webb has demonstrated its capability when it comes to obtaining spectra from distant exoplanets. This will allow astronomers to characterize exoplanet atmospheres and determine if they are capable of supporting life. As Cacciupoti said, the proximity of HOI 411, the fact that it is a Sun-like star, and its multi-planet system make it a prime target for follow-up studies by the JWST:

“Not knowing which planets actually have atmospheres, our job can be to look for good candidates to search them in the first place. To do so, we use a combination of information (on the star, the planet, and its orbit) to predict the strength of the signal of the atmosphere as seen by JWST (assuming the planet has one!). Planet TOI-411 c has one of the highest predicted strengths for a possible atmospheric signal among the planets of its kind (that we call Sub-Neptunes due to their sizes) and orbiting a Solar-like star.”

Further Reading: arXiv