Do we live in an unusual star system? Astronomers have been trying to figure out if the Solar System is unique compared to other stars and their orbiting planets, and they just took a major step towards answering the longstanding cosmic mystery.
An international team of planet-hunting astronomers spent three years analyzing thousands of measurements to calculate the masses of 120 confirmed exoplanets, as well as six candidate planets, spread across the northern sky. The result is an extensive catalog that details just how bizarre and exotic these alien worlds are and how they compare to our own star system. The findings are detailed in The Astrophysical Journal Supplement.
Using NASA’s Transiting Exoplanet Survey Satellite (TESS) in collaboration with the W.M. Keck Observatory on Maunakea, Hawai’i, the newly released catalog features a wide array of planets orbiting different kinds of stars. “With this information, we can begin to answer questions about where our solar system fits in to the grand tapestry of other planetary systems,” Stephen Kane, University of California, Riverside astrophysicist and principal investigator of the TESS-Keck Survey, said in a statement.
One of the oddballs in the catalog is a sub-Neptune with an unusually high density, making it both extremely massive and surprisingly small. Designated TOI-1824 b, this exoplanet is nearly 19 times the mass of Earth, but only 2.6 times the size of our home planet, meaning it holds a significant amount of mass in a tightly-squeezed body. “Planets similar in size typically have a mass between roughly 6 and 12 times the mass of Earth,” explained Joseph Murphy, a graduate student at the UC Santa Cruz and co-author of the study. This “exoplanet oddity” as Murphy refers to it, may have an Earth-like core surrounded by an unusually thin, hydrogen-dominated atmosphere, or it could have a water-rich core beneath a steam atmosphere.
Another extreme example is a super-Earth that is so close to its host star, it completes one orbit in less than 12 hours. TOI-1798 c orbits an orange dwarf star, along with another planet, TOI-1798 b, a sub-Neptune that has an orbit of about eight days.
“While the majority of planets we know about today orbit their star faster than Mercury orbits the Sun, USPs [ultra-short periods] take this to the extreme,” University of Kansas Physics and Astronomy graduate student Alex Polanski, the lead author of the paper, said in a statement. “TOI-1798 c orbits its star so quickly that one year on this planet lasts less than half a day on Earth.”
The super-Earth has likely lost any atmosphere that it had initially formed due to its extreme environment and close exposure to its star’s radiation.
The team of astronomers also discovered two new planets orbiting a star like our Sun, one of which is a sub-Saturn planet with a mass and radius that are somewhere between those of Neptune and Saturn. TOI-1386 b, as it’s known, only takes 26 days to orbit its star, while its neighboring planet, with a mass close to that of Saturn, takes 227 days to orbit the same star.
“There is ongoing debate about whether sub-Saturn planets are truly rare, or if we are just bad at finding planets like these,” Michelle Hill, UCR graduate student and lead author of a related survey paper, said in a statement. “So, this planet, TOI-1386 b, is an important addition to this demographic of planets.”
The planets themselves are not visible in the data, but they do have a visible effect on their host stars. During their orbits, the planets tug on their host stars, causing the stars to wobble. Astronomers can see the stars wobbling because their visible light turns slightly bluer when they move towards a telescope, and the light shifts slightly more red when they move away from the telescope. This is known as the Doppler effect, when there’s change in the frequency of sound, light, or other waves from a source that’s moving in relation to its observer.
Using radial velocity (RV) measurements, astronomers can not only detect an exoplanet orbiting a star but also learn more about the planet’s properties, including its mass. With ongoing surveys of the skies, we can expect more catalogs like this one that showcase the bizarre features of surrounding alien worlds.
“There are still thousands of unconfirmed planets from the TESS mission alone, so large releases of new planets like this will become more common as astronomers work to get a handle on the diversity of worlds we see today,” Ian Crossfield, an associate professor of physics and astronomy at the University of Kansas, and co-author of the paper, said in a statement.