Exoplanets – worlds of other suns

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A Mini-Neptune Orbiting the Metal-poor K Dwarf BD+29 2654
https://arxiv.org/abs/2306.08179

Two sub-Neptunes around the M dwarf TOI-1470
https://arxiv.org/abs/2306.08516

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Webb Telescope is powerful enough to see a variety of biosignatures in exoplanets, argues new paper

by Brian Koberlein, Universe Today

https://phys.org/news/2023-06-webb-tele ... tures.html

The best hope for finding life on another world isn't listening for coded messages or traveling to distant stars, it's detecting the chemical signs of life in exoplanet atmospheres. This long hoped-for achievement is often thought to be beyond our current observatories, but a new study argues that the James Webb Space Telescope (JWST) could pull it off.

Most of the exoplanets we've discovered so far have been found by the transit method. This is where a planet passes in front of its star from our point of view. Even though we can't observe the planet directly, we can see the star's brightness dip by a fraction of a percent. As we watch stars over time, we can find a regular pattern of brightness dips, indicating the presence of a planet.

The star dips in brightness because the planet blocks some of the starlight. But if the planet also has an atmosphere, there is a small amount of light that will pass through the atmosphere before reaching us. Depending on the chemical composition of the atmosphere, certain wavelengths will be absorbed, forming absorption spectra within the spectra of the starlight. We have long been able to identify atoms and molecules by their absorption and emission spectra, so in principle, we can determine a planet's atmospheric composition with the transit method.

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Webb rules out thick carbon dioxide atmosphere for rocky exoplanet
https://phys.org/news/2023-06-webb-thic ... phere.html
by NASA

Infrared measurements of TRAPPIST-1 c indicate that it is probably not as Venus-like as once imagined.

NASA's James Webb Space Telescope has successfully measured the heat radiating from TRAPPIST-1 c, an exoplanet orbiting a red dwarf star 40 light-years from Earth. With a dayside temperature of about 225 degrees Fahrenheit, it is the coolest rocky planet ever characterized using this method.

Unfortunately for those hoping that the TRAPPIST-1 system is a true analog to our own, the results are a bit disappointing. While TRAPPIST-1 c is roughly the same size and mass as Venus and receives the same amount of radiation from its star, it appears unlikely to have the same thick carbon dioxide atmosphere. This indicates that the planet, and perhaps the system as a whole, may have formed with very little water. The result is the latest in the quest to determine whether planetary atmospheres can survive the violent environs of a red dwarf star.

An international team of researchers has used NASA's James Webb Space Telescope to calculate the amount of heat energy coming from the rocky exoplanet TRAPPIST-1 c. The result suggests that the planet's atmosphere—if it exists at all—is extremely thin.

With a dayside temperature of roughly 380 Kelvin (about 225 degrees Fahrenheit), TRAPPIST-1 c is now the coolest rocky exoplanet ever characterized based on thermal emission. The precision necessary for these measurements further demonstrates Webb's utility in characterizing rocky exoplanets similar in size and temperature to those in our own solar system.

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New era of exoplanet discovery begins with images of 'Jupiter's younger sibling'
https://phys.org/news/2023-06-era-exopl ... piter.html
by W. M. Keck Observatory

Astronomers using W. M. Keck Observatory on Maunakea, Hawaiʻi Island have discovered one of the lowest-mass planets whose images have been directly captured. Not only were they able to measure its mass, but they were also able to determine that its orbit is similar to the giant planets in our own solar system.

The planet, called AF Lep b, is among the first ever discovered using a technique called astrometry; this method measures the subtle movements of a host star over many years to help astronomers determine whether hard-to-see orbiting companions, including planets, are gravitationally tugging at it.

The study, led by astronomy graduate student Kyle Franson at the University of Texas at Austin (UT Austin), is published in The Astrophysical Journal Letters.

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A Transiting Super-Earth in the Radius Valley and An Outer Planet Candidate Around HD 307842
https://arxiv.org/abs/2306.14655

Quote :

We report the confirmation of a TESS-discovered transiting super-Earth planet orbiting a mid-G star, HD 307842 (TOI-784). The planet has a period of 2.8 days, and the radial velocity (RV) measurements constrain the mass to be 9.67+0.83/-0.82 [Earth Masses]. We also report the discovery of an additional planet candidate on an outer orbit that is most likely non-transiting. The possible periods of the planet candidate are approximately 20 to 63 days, with the corresponding RV semi-amplitudes expected to range from 3.2 to 5.4 m/s and minimum masses from 12.6 to 31.1 [Earth Masses]. The radius of the transiting planet (planet b) is 1.93+0.11/-0.09 [Earth Radii], which results in a mean density of 7.4+1.4/-1.2 g/cm^3 suggesting that TOI-784b is likely to be a rocky planet though it has a comparable radius to a sub-Neptune. We found TOI-784b is located at the lower edge of the so-called ``radius valley'' in the radius vs. insolation plane, which is consistent with the photoevaporation or core-powered mass loss prediction. The TESS data did not reveal any significant transit signal of the planet candidate, and our analysis shows that the orbital inclinations of planet b and the planet candidate are 88.60+0.84/-0.86 degrees and <= 88.3-89.2 degrees, respectively. More RV observations are needed to determine the period and mass of the second object, and search for additional planets in this system.

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Astronomers find a planet that shouldn't exist
https://phys.org/news/2023-06-astronome ... uldnt.html
by University of Hawaii at Manoa

When our sun reaches the end of its life, it will expand to 100 times its current size, enveloping the Earth. Many planets in other solar systems face a similar doom as their host stars grow old. But not all hope is lost, as astronomers from the University of Hawaiʻi Institute for Astronomy (UH IfA) have made the remarkable discovery of a planet's survival after what should have been certain demise at the hands of its sun.

The Jupiter-like planet 8 UMi b, officially named Halla, orbits the red giant star Baekdu (8 UMi) at only half the distance separating the Earth and the sun. Using two Maunakea Observatories on Hawaiʻi Island—W. M. Keck Observatory and Canada-France-Hawaiʻi Telescope (CFHT)—a team of astronomers led by Marc Hon, a NASA Hubble Fellow at UH IfA, has discovered that Halla persists despite the normally perilous evolution of Baekdu.

Using observations of Baekdu's stellar oscillations from NASA's Transiting Exoplanet Survey Satellite (TESS), they found that the star is burning helium in its core, signaling that it had already expanded enormously into a red giant star once before. The work is published in the journal Nature.

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The TESS-Keck Survey. XVI. Mass Measurements for 12 Planets in Eight Systems
https://arxiv.org/abs/2306.16587

Quote :

With JWST's successful deployment and unexpectedly high fuel reserves, measuring the masses of sub-Neptunes transiting bright, nearby stars will soon become the bottleneck for characterizing the atmospheres of small exoplanets via transmission spectroscopy. Using a carefully curated target list and more than two years' worth of APF-Levy and Keck-HIRES Doppler monitoring, the TESS-Keck Survey is working toward alleviating this pressure. Here we present mass measurements for 11 transiting planets in eight systems that are particularly suited to atmospheric follow-up with JWST. We also report the discovery and confirmation of a temperate super-Jovian-mass planet on a moderately eccentric orbit. The sample of eight host stars, which includes one subgiant, spans early-K to late-F spectral types (Teff= 5200--6200 K). We homogeneously derive planet parameters using a joint photometry and radial velocity modeling framework, discuss the planets' possible bulk compositions, and comment on their prospects for atmospheric characterization.

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KMT-2022-BLG-0475Lb and KMT-2022-BLG-1480Lb: Microlensing ice giants detected via non-caustic-crossing channel
https://arxiv.org/abs/2307.00753

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Two new sub-Neptunes orbiting nearby stars discovered with TESS
https://phys.org/news/2023-07-sub-neptu ... -tess.html

by Tomasz Nowakowski , Phys.org

An international team of astronomers reports the discovery of two new sub-Neptune exoplanets using NASA's Transiting Exoplanet Survey Satellite (TESS). The newfound alien worlds, designated TOI-2084 b and TOI-4184 b, orbit nearby M-dwarf stars and are about two and a half times larger than the Earth. The finding was detailed in a paper published June 26 on the preprint server arXiv.

TESS is conducting a survey of about 200,000 of the brightest stars near the sun with the aim of searching for transiting exoplanets. So far, it has identified nearly 6,700 candidate exoplanets (TESS Objects of Interest, or TOI), of which 360 have been confirmed so far.

Recently, a group of astronomers led by Khalid Barkaoui of the University of Liège in Belgium, has confirmed another two TOI exoplanet candidates. Using ground-based facilities, they conducted follow-up observations of two stars designated TOI-2084 and TOI-4184, finding that transit signals in their light curves are of planetary nature.