What is TOI-561b? 50% larger than Earth, this Super-Earth is one of the oldest rocky planets yet discovered
One of the oldest stars in the Milky Way galaxy hosts an unusually hot and rocky “Super-Earth” planet. Known as TOI-561b, the planet orbits the star TESS Object of Interest (TOI) 561, named for the ongoing NASA Transiting Exoplanet Survey Satellite or TESS planet-hunting mission.
The planet is unlike any other Super-Earth found to date, according to scientists. It is about 50% larger than Earth but requires less than half a day to orbit its star. According to experts, for every day on Earth, this planet orbits its star twice.
Part of the reason for the short orbit is the planet’s proximity to its star, which also creates incredible heat. Its estimated average surface temperature is over 2,000 degrees Kelvin — much too hot to host life as we know it today, though it may once have been possible, reveals analysis published in the Astronomical Journal.
“The rocky planet orbiting TOI-561 is one of the oldest rocky planets yet discovered. Its existence shows that the universe has been forming rocky planets almost since its inception 14 billion years ago,” says Lauren Weiss, Beatrice Watson Parrent Postdoctoral Fellow at the University of Hawaiʻi Institute for Astronomy (IfA), and leader of the team that discovered the TOI-561 planetary system.
What are these planets?
Super-Earth is a reference only to an exoplanet’s size – it is the name given to planets that are larger than Earth and smaller than Neptune – and does not imply that they are necessarily similar to our home planet. Super-Earths can be up to 10 times more massive than Earth.
“Super-Earths, a class of planets unlike any in our solar system, are more massive than Earth yet lighter than ice giants like Neptune and Uranus and can be made of gas, rock, or a combination of both. They are between twice the size of Earth and up to 10 times its mass,” explains NASA. It adds, “The true nature of these planets remains shrouded in uncertainty because we have nothing like them in our own solar system – and yet, they are common among planets found so far in our galaxy.”
Observations from NASA’s Spitzer Space Telescope led to the first temperature map of a Super-Earth in 2016. 55 Cancri e (or CNC e) is a rocky planet and is nearly two times as big as ours. The analysis shows extreme temperature swings from one side of the planet to the other, and hints that a possible reason for this is the presence of lava flows.
According to NASA, the fact Spitzer found the night side to be significantly colder than the day side means heat is not being distributed around the planet very well. “The data argues against the notion that a thick atmosphere and winds are moving heat around the planet as previously thought. Instead, the findings suggest a planet devoid of a massive atmosphere, and possibly hint at a lava world where the lava would become hardened on the night side and unable to transport heat,” the findings state.
What does the latest study reveal?
TOI-561 has at least two other planets transiting the star, both of which have about twice Earth’s radius and are too large and low-mass to be rocky. According to investigators, TOI-561 belongs to a rare population of stars called the galactic thick disk. Stars in this region are chemically distinct, with fewer heavy elements such as iron or magnesium that are associated with planet building.
“Thick disk stars are chemically distinct, with fewer trace heavy elements (and especially less iron) than typical stars of the Milky Way, suggesting they formed early, approximately 10 billion years ago. They also have wandering motions that can lift them out of the galactic plane, providing an epic view of our own spiral galaxy,” explains researchers.
The TESS mission team used the University of California’s access to the WM Keck Observatory in Hawaii to confirm the presence of planet TOI-561b. The observatory’s equipment also helped them calculate the planet’s mass, density and radius. The rocky planet orbiting TOI-561 transits its star, meaning that the planet passes in front of its star as seen from Earth, blocking a fraction of the starlight. Since the planet is small, with a radius only one-and-a-half times that of Earth, the reduction of light it causes is miniscule — just 0.025% of the star’s brightness.
While the planet has roughly three times the mass of Earth, its density was calculated to be the same as our planet. “This is surprising because you’d expect the density to be higher. This is consistent with the notion that the planet is extremely old,” states Stephen Kane, University of California Riverside planetary astrophysicist and team member.
The older a planet is, the less dense it is likely to be because not as many heavy elements were available when it formed, explains Kane. Heavy elements are produced by fusion reactions in stars as they age. Eventually, stars explode, dispersing these elements from which new stars and planets will form.
Astronomers are continually trying to understand the relationship between the mass and radius of the planets they find. This information throws light regarding the interior structure of planets that with today’s technology are too far away to visit and sample. Combining the mass with the radius determined from the transits, the researchers concluded that TOI-561b is most likely rocky, perhaps with less iron than Earth.
“Information about a planet’s interior gives us a sense of whether the surface of the planet is habitable by life as we know it. Though this particular planet is unlikely to be inhabited today, it may be a harbinger of many rocky worlds yet to be discovered around our galaxy’s oldest stars,” emphasizes Kane.