Moon-forming disc around exoplanet + Astrophysicist: 1st GW observatory on Moon

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Astronomers make first clear detection of a moon-forming disc around an exoplanet

RELEASE: Using the Atacama Large Millimetre/submillimeter Array (ALMA), in which the European Southern Observatory (ESO) is a partner, astronomers have unambiguously detected the presence of a disc around a planet outside our Solar System for the first time. The observations will shed new light on how moons and planets form in young stellar systems.

"Our work presents a clear detection of a disc in which satellites could be forming," says Myriam Benisty, a researcher at the University of Grenoble, France, and at the University of Chile, who led the new research published today in The Astrophysical Journal Letters. "Our ALMA observations were obtained at such exquisite resolution that we could clearly identify that the disc is associated with the planet and we are able to constrain its size for the first time," she adds.

The disc in question, called a circumplanetary disc, surrounds the exoplanet PDS 70c, one of two giant, Jupiter-like planets orbiting a star nearly 400 light-years away. Astronomers had found hints of a "moon-forming" disc around this exoplanet before but, since they could not clearly tell the disc apart from its surrounding environment, they could not confirm its detection -- until now.

In addition, with the help of ALMA, Benisty and her team found that the disc has about the same diameter as the distance from our Sun to the Earth and enough mass to form up to three satellites the size of the Moon.

But the results are not only key to finding out how moons arise. "These new observations are also extremely important to prove theories of planet formation that could not be tested until now," says Jaehan Bae, a researcher from the Earth and Planets Laboratory of the Carnegie Institution for Science, USA, and author on the study.

Planets form in dusty discs around young stars, carving out cavities as they gobble up material from this circumstellar disc to grow. In this process, a planet can acquire its own circumplanetary disc, which contributes to the growth of the planet by regulating the amount of material falling onto it. At the same time, the gas and dust in the circumplanetary disc can come together into progressively larger bodies through multiple collisions, ultimately leading to the birth of moons.

But astronomers do not yet fully understand the details of these processes. "In short, it is still unclear when, where, and how planets and moons form," explains ESO Research Fellow Stefano Facchini, also involved in the research.

"More than 4000 exoplanets have been found until now, but all of them were detected in mature systems. PDS 70b and PDS 70c, which form a system reminiscent of the Jupiter-Saturn pair, are the only two exoplanets detected so far that are still in the process of being formed," explains Miriam Keppler, researcher at the Max Planck Institute for Astronomy in Germany and one of the co-authors of the study [1].

"This system therefore offers us a unique opportunity to observe and study the processes of planet and satellite formation," Facchini adds.

PDS 70b and PDS 70c, the two planets making up the system, were first discovered using ESO's Very Large Telescope (VLT) in 2018 and 2019 respectively, and their unique nature means they have been observed with other telescopes and instruments many times since [2].

The latest high resolution ALMA observations have now allowed astronomers to gain further insights into the system. In addition to confirming the detection of the circumplanetary disc around PDS 70c and studying its size and mass, they found that PDS 70b does not show clear evidence of such a disc, indicating that it was starved of dust material from its birth environment by PDS 70c.

An even deeper understanding of the planetary system will be achieved with ESO's Extremely Large Telescope (ELT), currently under construction on Cerro Armazones in the Chilean Atacama desert. "The ELT will be key for this research since, with its much higher resolution, we will be able to map the system in great detail," says co-author Richard Teague, a researcher at the Center for Astrophysics | Harvard & Smithsonian, USA. In particular, by using the ELT's Mid-infrared ELT Imager and Spectrograph (METIS), the team will be able to look at the gas motions surrounding PDS 70c to get a full 3D picture of the system.


[1] Despite the similarity with the Jupiter-Saturn pair, note that the disc around PDS 70c is about 500 times larger than Saturn's rings.

[2] PDS 70b was discovered using the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument, while PDS 70c was found using the VLT’s Multi Unit Spectroscopic Explorer (MUSE). The two-planet system has been investigated using the X-shooter instrument too, also installed on ESO’s VLT.

Astrophysicist outlines ambitious plans for the first gravitational wave observatory on the moon

RELEASE: The Idea. Vanderbilt astrophysicist Karan Jani has led a series of studies that make the first case for a gravitational wave infrastructure on the surface of the moon. The experiment, dubbed Gravitational-Wave Lunar Observatory for Cosmology, uses the moon's environment and geocentric orbit to analyze mergers of black holes, neuron stars and dark matter candidates within almost 70 percent of the entire observable volume of the universe, he said.

"By tapping into the natural conditions on the moon, we showed that one of the most challenging spectrum of gravitational waves can be measured better from the lunar surface, which so far seems impossible from Earth or space," Jani said.

Why it matters. "The moon offers an ideal backdrop for the ultimate gravitational wave observatory, since it lacks an atmosphere and noticeable seismic noise, which we must mitigate at great cost for laser interferometers on Earth," said Avi Loeb, professor of science at Harvard University and bestselling author of books about black holes, the first stars, the search for extraterrestrial life and the future of the universe. "A lunar observatory would provide unprecedented sensitivity for discovering sources that we do not anticipate and that could inform us of new physics. GLOC could be the jewel in the crown of science on the surface of the moon."

This work comes as NASA revives its Artemis program, which aims to send the first woman and the next man to the moon as early as 2024. Ongoing commercial work by aerospace companies, including SpaceX and BlueOrigin, also has added to the momentum behind planning for ambitious scientific infrastructure on the surface of the moon.

What's next. "In the coming years, we hope to develop a pathfinder mission on the moon to test the technologies of GLOC," Jani said. "Unlike space missions that last only a few years, the great investment benefit of GLOC is it establishes a permanent base on the moon from where we can study the universe for generations, quite literally the entirety of this century." Currently the observatory is theoretical, with Jani and Loeb receiving a strong endorsement from the international gravitational-wave community.

"It was a great privilege to collaborate with an innovative young thinker like Karan Jani," Loeb said. "He may live long enough to witness the project come to fruition."
Yazata Offline
European Southern Observatory photo

The bright circle appears to be a planetary accretion disk where plants are forming. The central object is a young star. And the little object to its right appears to be a newly formed exoplanet which in turn appears to have a little accretion disk of its own. presumably where moons are forming.

[Image: eso2111a.jpg]

[Image: eso2111a.jpg]

And regarding a GW observatory on the Moon, the Apollo astronauts left a seismograph on the lunar surface. It failed to detect any Moon vibrations associated with GWs, but did succeed in placing an upper bound on their intensity.

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