To detect exoplanets, astronomers rely mainly on two approaches: the transit method and radial velocity measurements. Transits are detected when a planet dims its star's light by passing in front of it, while radial velocity captures the slight wobble in a star's motion caused by orbiting planets. Although the transit method is highly efficient, scanning thousands of stars simultaneously, it can yield many false positives. Hence, radial velocity observations are essential for verifying candidates and determining their masses.
"Over the past two decades, the transit method gained prominence," explained Mara Attia, a postdoctoral researcher in UNIGE's Department of Astronomy and co-author of the study. "However, the transit method produces many false positives. 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."
Together, these two techniques perform a complementary observational dance. Transit data from missions like NASA's TESS provide candidate planets, which are then examined with instruments like ESPRESSO to confirm their planetary status and deduce their physical characteristics.
Launched in 2018, TESS has delivered over 7,000 potential exoplanet detections. ESPRESSO, recognized as the most accurate spectrograph available, plays a vital role in verifying and studying these candidates. One of ESPRESSO's projects, led by UNIGE's Department of Astronomy, focuses specifically on confirming TESS discoveries and exploring the boundary between rocky and gaseous planets.
In our solar system, rocky planets like Earth and Venus are dwarfed in mass by gas giants such as Uranus and Neptune. However, data from other star systems reveal that planets with masses between three and ten times that of Earth are common. These bodies, depending on their density, are termed either super-Earths (likely rocky) or mini-Neptunes (likely gaseous), due to the uncertainty surrounding their exact compositions.
While exoplanets abound in the galaxy, identifying and confirming each one remains a time-intensive endeavor. The discovery of TOI-512 b, for instance, spanned multiple years: 72 days of TESS monitoring over a two-year stretch, followed by 37 nights of ESPRESSO observations across eight months, and concluded with meticulous analysis by the research team.
TOI-512 b boasts a mass 3.5 times that of Earth and a radius 1.5 times greater. A cubic centimeter of its material weighs 5.62 grams, closely resembling Earth's own density. Orbiting a K-type star with a surface temperature near 5000 degrees Celsius, the planet completes a full orbit in just over seven days. Its characteristics place it squarely within the transitional zone between rocky and gaseous exoplanets.
"The precision of ESPRESSO has been crucial in characterizing the composition of TOI-512 b. 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. Many more will be needed to transform our hypotheses into scientific certainties," concluded Jose Rodrigues, a doctoral researcher at the Porto Institute of Astrophysics and lead author of the study.
Research Report:TOI-512: Super-Earth transiting a K-type star discovered by TESS and ESPRESSO
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