Almost everyday, the number of confirmed exoplanet discoveries grows.
The majority of those planets, which sit just above 5,500 in total, have been identified by the Kepler space telescope. But for the last few years, NASA's Transiting Exoplanet Survey Satellite (TESS) has been steadily adding new alien worlds to our growing planetary catalog of the cosmos.
Using a statistical method to comb through TESS's large quantities of data on the night sky, a group of scientists led by Priyashkumar Mistry, a Ph.D. student at the University of New South Wales, have reported on the potential discovery of eight new exoplanets. What's more, each one of these planets is considered to be a "super-Earth," a class of exoplanet that is larger than Earth but smaller than Neptune, according to NASA.
To date, TESS has confirmed almost 400 exoplanets, and yet another 6,977 await confirmation. The satellite observes nearby stars, waiting for dips, or fluctuations in the brightness of the stars. Such dips indicate to astronomers that something likely passed in between us and the star — and that something could be a new exoplanet.
"If this orbital motion ever comes between us and the star we will observe a dip in the brightness of that observed star. This is what we call a transit," Mistry told Space.com.
Mistry and his team used the Validation of Transiting Exoplanets using Statistical Tools (VaTEST) project to identify anomalies, which could indicate the presence of exoplanets, in TESS's data.
Why do we need statistical tools?
It's not only the case that these dips are caused by transiting exoplanets. Such false positives, which could include a star orbiting another star (binary system), or a background source, could generate a transit-like signal.
Mistry explains that the transit method can only provide the radius of an orbiting body. What if a planet sized star, such as a brown or red dwarf is in orbit? Astronomers would usually work out the mass of a transiting object using a method called radial velocity (RV), which is where an orbiting body exerts a gravitational pull on its home star. This results in the star doing a little dance, or 'wobble'.
To detect the RV signal, though, it can take a lot of time observing just one star, especially if the exoplanet has a long orbital period — time and resources that the researchers didn't have.
However, the VaTEST provided Mistry and his team with another means of confirming whether these transiting events were actually the result of orbiting exoplanets.
"The tool takes in the transit data and some inputs such as transit depth, period, TESS identifier, etc. Then based on that it starts fitting different models on the data and performs some probability calculations. And then finally it calculates False Positive Probability (FPP), if it turns out to be < 1% then we can validate that transit signal as a planetary transit," Mistry says.
The statistical tool calculated that eight such transiting events were likely caused by a class of exoplanet that astronomers call 'super-Earths' — and that six of them fall into the region known as 'keystone planets,' which have characteristics that help astronomers better understand the overall exoplanet population. This makes them highly attractive for further study.
What is a keystone planet?
To understand what astronomers mean by 'keystone planet', we first have to understand the radius valley concept. The radius valley reveals a scarcity of planets between 1.5 and 2 Earth radii, with orbital periods less than 100 days in the known population of exoplanets that orbit low-mass and sun-like stars. This radius range covers super-Earths, and another class of planet — sub-Neptunes, which are exoplanets with a smaller radius than Neptune.
Why does this scarcity exist? Some theories suggest this may be due to photoevaporation mass loss, where intense radiation from a home star could gradually strip away a planet's atmosphere over time. This suggests that planets within this keystone region should be predominantly rocky, but observations are yet to confirm whether this is in fact the case.
"To understand this contradiction we need more and more keystone region planets. And that's the reason why our validated exoplanets are interesting to study," explains Mistry. Adding more keystone planets to our exoplanet catalog, with the potential to do follow up observations with the James Webb Space Telescope (JWST), should help astronomers resolve what explanation best fits this mystery in the exoplanet data.
Could one of these super-Earth's be a home to life?
While super-Earth's do have the name 'Earth' in their title, this doesn't necessarily refer to their life-giving potential. Rather, life would likely find it hard to establish itself on any of the super-Earths discovered by Mistry and his team. The reason being the close proximity these planets have to their home stars.
"They are closer to their host star than Mercury is from the sun," says Mistry. This usually means they are tidally locked, where one side of the planet is forever facing the star, with the other side cloaked in eternal darkness. In this sense, it's either scorching or freezing temperatures, neither of which are particularly life-friendly.
"But who knows. The cosmos is filled with so many surprises," Mistry quips.
A study of the eight super-Earths can be found on the preprint server arXiv, with the paper currently under review at the Publications of the Astronomical Society of Australia.
