Gas giants remain key to understanding exoplanetary systems, says leading French astrophysicist


the artist’s interpretation of an alien planet system

On top of a hill overlooking the Côte d’Azur, a few astronomers gather for lunch. Although well known for its rich and famous and for a large number of Hollywood stars who annually descend on the Cannes Film Festival just below the coast, the Côte d’Azur has been home to four internationally renowned optical observatories. . administrative offices here at the Nice Observatory.

In a straight line, Nice is also a few hundred km from the Haute Provence Observatory, where in 1995 Swiss Nobel Prize-winning astronomers Michel Mayor and Didier Queloz surprised the world by tracing a bizarre object. of Jupiter around the nearby sun. star 51 Pegasus.

This afternoon, however, the chef of the observatory’s restaurant approaches Tristan Guillot, a long-standing extrasolar planet hunter, and teases him about whether he will want his coffee at the same time as his dessert, which in tighter French circles could be considered a false step. But Guillot, a planetary scientist and astrophysicist at the Côte d’Azur Observatory, takes cordiality with a characteristic good humor.

Guillot is one of the many unknown heroes of the exoplanet revolution of the last quarter of a century in which our planetary paradigm has gone from a census of exactly one — our Goldilocks-like solar system — to about 5,000 confirmed extrasolar solar systems. . Almost none of which resembles ours.

Last week I went to lunch to discuss how far exoplanetary science has come since those dive days in the mid-1990s.

Guillot spent two years in the mid-1990s as a postdoctoral researcher at the University of Arizona in Tucson, where he was part of a team — with longtime planetary researchers Adam Burrows and William Hubbard — that pioneered models. atmospheric of giant extrasolar gas planets. These models are crucial to understanding these bizarre exosystems.

“We published an article in the magazine Nature about the expected luminosities of the giant extrasolar planets a few months before 51 Pegasi b was discovered, ”Guillot told me. “But we were thinking about relatively young planets far away from the star, so it was a big surprise to find a planet so close to the star.”

Guillot’s doctoral dissertation focused on the internal structure of Jupiter and Saturn. Therefore, once Mayor and Queloz detected this strange half-massive planet from Jupiter 51 Pegasi b, Guillot realized that by studying the four gas giants of Jupiter, Saturn and our own solar system, he could make a vital contribution to understanding of extrasolar planetary systems.

Surprisingly, none of the currently known ecosystems appear to be very similar to our own solar system. In fact, after about 25 years of research, it is puzzling and surprising that we have not found a planetary system that, dynamically, compositionally or architecturally is really a 2.0 solar system.

Twenty-five years ago, astronomers only detected little-hung planetary fruit, hot and wild Jupiters in short orbits around their parent stars, because that’s what most sensitized us to find in our Doppler spectroscopy surveys. ground-based. As a planet revolves around its star in a short orbit, it can gravitationally perturb its parent star to such an extent that the effects can be measured at the radial velocity of a given star. That is, their movement toward or away from us along our line of sight. This causes what is known as the center of gravity of a star to rotate on the order of tens of meters per second. Our own Jupiter is even prone to such an effect in our Sun.

Thus, using this method, Mayor and Queloz were able to detect the gravitational effects of a planet half the mass of Jupiter at Pegasus 51, located about 50 light-years away from the northern constellation Pegasus.

The good news is that these strange hot systems of Jupiter account for less than 4 percent of all known planetary systems.

More recently, Guillot has focused primarily on our own solar system using data collected from NASA’s Juno orbital mission to Jupiter, which has now been expanded to 2025. One of Juno’s goals was to place deep restrictions. composition of Jupiter.

“We’ve discovered that Jupiter and the giant planets aren’t as simple as we thought,” Guillot said. “The central core of the planet is probably not a compact core and is diluted in a surrounding envelope with stable regions in that envelope.”

As it deepens on Jupiter, the gas transforms into something more like a fluid, Guillot says. Even when it reaches the inner metallic hydrogen envelope of the planet, the composition remains that of a fluid, says Guillot. Near the center of Jupiter, the planet is ionized by two to four million terrestrial atmospheres of pressure, he says. That is why Jupiter has such a high electrical conductivity that Guillot says it is also likely to be the source of the planet’s extremely robust magnetic field.

Guillot is part of a French team of astronomers that has used the most isolated astronomical observatory on Earth – an Antarctic ice station that houses a 40 cm optical telescope – to confirm many of the planetary detections that the TESS (Transiting ) from the NASA Exoplanet Survey Satellite) has been doing.

The Antarctic Search for Transiting ExoPlanets (ASTEP) project is robotic, but it is part of the Concordia Franco-Italian base with permanent staff at Dome C, a 3,200-meter ice climb about 1,000 km from the geographic south pole. Due to its three-month night during the Antarctic winter, good weather conditions and minimal atmospheric disturbance, ASTEP can make terrestrial photometric observations that approach space-based quality.

This year we were able to observe from Earth the first circumbinary planet; a planet orbiting two stars like Tatooine (in Star Wars), says Guillot. We also look for planets in long orbital periods that transit their stars, which are much harder to detect, he says.

While gas giant planets in any strip are unlikely to harbor any kind of life, their study continues to provide information about planetary systems in general.

Giant planets had a huge impact on the architecture of our solar system and the flow of comets going into our inner solar system, says Guillot. According to him, it depends on the positive or negative system.

Therefore, something as mundane as lunch in one of the world’s historical observatories — windows to the true search for understanding ourselves — takes on a new meaning. An act as everyday as serving a plate of hake fish in curry pauses to marvel at how far this planet has come since it was launched into the protoplanetary disk of our solar system.

How unique is our own solar system?

“At the moment, it’s still hard to say,” Guillot said.

In general, the exoplanetary systems we are discovering are quite different from ours and show enormous diversity, he says.

“I don’t think we have a twin of our own solar system in the sample; therefore, our solar system must be quite rare, ”said Guillot.

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