The study, part of the TESS-Keck Survey, is published in The Astrophysical Journal Supplement.

"These two facilities (TESS and Keck) work synergistically to study exoplanets," said lead author Alex Polanski, a doctoral student in physics and astronomy at KU. "TESS is a satellite orbiting above Earth's atmosphere, scanning the sky for exoplanets using the transit method."

The transit method involves observing a planet as it passes in front of its host star, causing a slight dimming of the star's light.

"By detecting and measuring these transit events, researchers can determine the planet's orbital period and size," Polanski said. "Larger planets block more light, making them easier to detect. However, transit data doesn't provide information about the planet's mass, which is crucial for understanding its composition."

To determine the mass of exoplanets, researchers used the Keck Observatory to execute a technique called "radial velocity," according to Polanski.

"This method involves observing the host star and measuring its spectrum," he said. "Stars contain elements like hydrogen, helium and iron, which create characteristic absorption lines in their spectra. As a planet orbits its star, the star experiences slight gravitational pull, causing it to wobble. This wobble shifts the star's spectral lines due to the Doppler effect. The amount of shift in the spectral lines is related to the planet's mass; larger planets cause greater shifts."

Polanski noted that radial velocity was used to discover the first exoplanets around sun-like stars in the 1990s, known as "hot Jupiters" - massive Jupiter-sized planets with short orbital periods. However, smaller planets, especially those between Earth and Neptune in size, create much smaller shifts and need more sensitive instruments like those at Keck.

"The TESS-Keck Survey represents the single largest contribution to understanding the physical nature and system architectures of new planets TESS has discovered," said Ian Crossfield, associate professor of physics and astronomy at KU, who co-wrote the new paper. "Catalogs like this help astronomers place individual worlds in context with the rest of the exoplanet population."

A global team of astronomers spent three years developing the catalog, analyzing 9,204 radial velocity measurements, 4,943 of which were recorded over 301 observing nights using Keck Observatory's High-Resolution Echelle Spectrometer.

"With this information, along with the planets' radii, scientists can estimate the planets' densities and infer their possible compositions," Polanski said. "This paper is the largest of its kind to date. The last similar one that came out was, I think, a sample of 27 exoplanets. This is up to 126 planets."

The team identified two planets - TOI-1824 b and TOI-1798 c - that might deepen understanding of planetary diversity and evolution:

TOI-1824 b: a superdense sub-Neptune
"At nearly 19 times the mass of Earth, but only 2.6 times the size of our home planet, TOI-1824 b is an exoplanet oddity," said co-author Joseph Murphy, a doctoral student at UC-Santa Cruz. "Planets similar in size typically have a mass between roughly 6 and 12 times the mass of Earth."

TOI-1798 c: a rare, extreme super-Earth
TOI-1798 is an orange dwarf star with two planets: TOI-1798 b, a sub-Neptune with an eight-day orbit, and TOI-1798 c, a super-Earth that orbits in less than 12 hours.

"USPs take this to the extreme," Crossfield said. "TOI-1798 c orbits its star so quickly that one year on this planet lasts less than half a day on Earth. Because of their proximity to their host star, USPs are also ultra-hot-receiving more than 3,000 times the radiation that Earth receives from the Sun. Existing in this extreme environment means that this planet has likely lost any atmosphere that it initially formed."

Polanski said better knowledge of exoplanets and their star systems would enhance understanding of our own solar system.

"Astronomy has gone through several phases of 'great demotions,'" he said. "However, our solar system might be more unique than we thought. About half of all Sun-like stars have a binary companion. Our Sun does not. Only about 10% of Sun-like stars have gas giants like Jupiter or Saturn. This suggests our solar system might be less typical than we assumed."

Research Report:The TESS-Keck Survey. XX. 15 New TESS Planets and a Uniform RV Analysis of All Survey Targets

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