The signature for the newly discovered planet was hiding in data from NASA’s Transiting Exoplanet Survey Satellite, or TESS. Using TESS data, scientists look for changes in brightness of nearby stars, which could indicate the presence of orbiting planets.
Jacobs is part of a group of citizen scientists who look at plots of TESS data, showing the change in a star’s brightness over time, in search of new planets. While professional astronomers use algorithms to scan tens of thousands of data points from stars automatically, these citizen scientists use a program called LcTools, created by Alan R. Schmitt, to inspect telescope data by eye. That’s why Jacobs’ group, which includes several citizen scientists and two veteran astronomers, calls themselves the Visual Survey Group. Many of them met while working on Planet Hunters, a NASA-funded citizen science project through Zooniverse that focused on data from NASA’s Kepler spacecraft.
On February 1, 2020, Jacobs happened to notice a plot showing starlight from TOI-2180 dim by less than half a percent and then return to its previous brightness level over a 24-hour period, which may be explained by an orbiting planet that is said to “transit” as it passes in front of the star from our point of view. By measuring the amount of light that dims as the planet passes, scientists can estimate how big the planet is and, in combination with other measurements, its density. But a transit can only be seen if a star and its planet line up with telescopes looking for them.
A graph showing starlight over time is called a “light curve.” The Visual Survey Group alerted two professional scientist collaborators — Paul Dalba at the University of California, Riverside, and Diana Dragomir, assistant professor at the University of New Mexico, that this light curve was potentially interesting.
“With this new discovery, we are also pushing the limits of the kinds of planets we can extract from TESS observations,” Dragomir said. “TESS was not specifically designed to find such long-orbit exoplanets, but our team, with the help of citizen scientists, are digging out these rare gems nonetheless.”
TOI-2180 b is almost three times more massive than Jupiter but has the same diameter, meaning it is more dense than Jupiter. This made scientists wonder whether it formed in a different way than Jupiter.
Another clue about the planet’s formation could be what’s inside it. Through computer models they determined that the new planet may have as much as 105 Earth masses worth of elements heavier than hydrogen and helium. “That’s a lot,” says Dalba. “That’s more than what we suspect is inside Jupiter.
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Astronomers still have much to learn about the range of planets that are out there. About 4,800 exoplanets have been confirmed, but there are thought to be billions of planets in our galaxy. The new finding indicates that among giant planets, some have many more heavy elements than others.
In our solar system, gigantic Jupiter orbits the Sun every 12 years; for Saturn, a “year” is 29 years. We don’t have giant planets like TOI-2180 b between the Earth and Sun. But outside the solar system, astronomers have found dozens of exoplanets that are even bigger than Jupiter and orbit much closer to their stars, even closer than the orbit of Mercury.
With an average temperature of about 170 degrees Fahrenheit, TOI-2180 b is warmer than room temperature on Earth, and warmer than the outer planets of our solar system including Jupiter and Saturn. But compared to the array of transiting giant exoplanets that astronomers have found orbiting other stars, TOI-2180 b is abnormally chilly.
“It's a nice stepping stone in between most giant exoplanets we’ve found, and then really cold Jupiter and Saturn,” Dalba said.
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