By Kurtis Archer | 2025-05-02

An international team including researchers from the University of Geneva (UNIGE) and the National Centre of Competence in Research (NCCR) PlanetS has discovered a super-Earth 218 light-years away that could help astronomers understand why the universe lacks certain exoplanets.

A super-Earth is a planet larger than Earth but smaller than Neptune, with a mass typically between 2 and 10 times that of Earth.

The study was led by José Rodrigues of the Porto Institute of Astrophysics in Portugal. The planet is named TOI-512b.

Confirming the facts

NASA's Transiting Exoplanet Survey Satellite (TESS) and the Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observation (ESPRESSO) instrument for measuring planets on the Very Large Telescope (VLT) in Chile made the discovery.

TESS has revealed more than 7,000 exoplanet candidates in the past seven years, and ESPRESSO is the most accurate spectrograph for measuring radial velocities in existence.

TESS observed TOI-512b for 72 days over more than two years, while ESPRESSO made observations for 37 nights over eight months, with several months of intense analysis performed by the team afterward.

TOI-512b was first discovered by TESS in 2020 as the planet made its transit in front of its star. Scientists used the amount of light blocked from the star to determine that TOI-512b's radius is 1.54 times larger than Earth's radius.

To confirm the planet is real and not a false positive, ESPRESSO measured the "wobble" in the star's rotation created by the planet's gravity, also known as the star's radial velocity. TOI-512b's mass is 3.57 times greater than Earth's mass, ESPRESSO determined.

Mara Attia, a postdoctoral fellow in the Department of Astronomy at UNIGE's Faculty of Science and a co-author of the study, explained the importance of using both techniques.

"Over the past two decades, the transit method gained prominence. However, the transit method produces many false positives," she said.

"It is often necessary to use the other technique to confirm definitively the presence of an exoplanet around a star. Nor does it allow us to measure a planet's mass, an essential variable for studying its nature. Here too, we have to use the radial velocity method."

Hypotheses

With the radius and mass of the planet known, astronomers calculated the average bulk density of TOI-512b at 5.62 grams per cubic cm -- 1.02 times that of Earth's 5.52 grams per cubic cm.

If TOI-512b were the size of Earth, the density would suggest a rocky planet like Earth. Because the planet is larger and more massive than Earth, the certainty of that composition decreases.

TOI-512b orbits its star about once every 7.2 days compared to Earth's 365, and at a distance of 9,863,797km. It is too near the star to be in its habitable zone, receiving 112 times the heat that Earth does from the sun.

Astronomers have noticed a relative lack of exoplanets with a radius between 1.8 and 2.4 times greater than Earth's radius. This range is known as the "hot Neptune desert."

One leading theory is that when Neptune-sized planets rich in gas and ice move inward toward their star, the atmosphere is blown away by stellar radiation. A given planet becomes a much smaller world than it once was.

Another possibility is core-powered mass loss -- a situation where heat from a planet's core leaks out into a thick layer of gas that gets pushed away from the planet. The planet then shrinks over time.

Using the density, mass and volume of the planet, Rodrigues and his team modeled what TOI-512b's interior might be.

The likeliest scenario based on their calculations is that the planet has a small inner core taking up 13% of the planet's mass, a mantle taking up 69% and a layer of water contributing up to 16%, with the now-depleted atmosphere making up 2%.

For comparison, Earth's inner core makes up 1.7% of its mass, the mantle is 67% and water on Earth takes up only 0.02% of the total mass. Earth's atmosphere's mass is negligible, and the remaining 31.3% consists of the planet's outer core and crust.

More research needed

Core-powered mass loss is the reason for TOI-512b's attributes, as stellar radiation reducing the size of the world would have resulted in no water or gas, the research team concluded.

This explanation also makes sense given the age of TOI-512b, as core-powered mass loss can take billions of years. TOI-512b is about 8.235 billion years old, give or take 4.386 billion years.

Scientists point out that the system's conditions could have resulted in both solar radiation and core-powered mass loss reducing the size of the planet, or perhaps some yet unknown process was the reason.

They hope the Armazones high Dispersion Echelle Spectrograph instrument on the forthcoming Extremely Large Telescope at the European Southern Observatory in Chile will be able to deliver more information about TOI-512b through transit spectroscopy -- which is when astronomers study a planet's atmosphere by analyzing the starlight that filters through it when the planet passes in front of its star.

"It's a small addition to the already long list of known planets, but such discoveries are essential to improve our understanding of the mechanisms of planet formation and evolution," Rodrigues said of the study. "Many more will be needed to transform our hypotheses into scientific certainties."