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Gas Giants Remain Key To Understanding Exoplanetary Systems, Says Leading French Astrophysicist

High on a hillside overlooking the French Riviera, a smattering of astronomers is gathering for their noonday meal. Although best known for the rich and famous and a coterie of Hollywood stars who annually descend on the Cannes Film Festival just a bit further down the coast, the Cote d’Azur has long been home to four internationally renowned optical observatories, all of which have administrative offices here at Nice Observatory.

As the crow flies, Nice also lies only a few hundred km from the Observatoire de Haute-Provence where in 1995, Nobel Prize-winning Swiss astronomers Michel Mayor and Didier Queloz shocked the world by tracking a bizarre Jupiter-like object around the nearby sunlike star 51 Pegasi. 

On this particular afternoon, however, the observatory restaurant’s chef approaches longtime extrasolar planet hunter Tristan Guillot and teases him about whether he’ll want his coffee at the same time as his dessert which in stricter French circles might be considered a faux pas. But Guillot, a planetary scientist and astrophysicist at l’Observatoire de la Cote d’Azur, takes the friendly ribbing with characteristic good humor.   

Guillot is one of the many unsung heroes of the last quarter century’s exoplanet revolution in which our planetary paradigm has shifted from a census of exactly one —- our own Goldilocks-like solar system —- to some 5,000 confirmed extrasolar solar systems. Almost none of which resemble our own.  

Last week, I dropped by for lunch to discuss just how far exoplanetary science has come since those heady 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 atmospheric models of extrasolar gas giant planets. Such models are crucial in understanding these bizarre exo-systems.

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

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

Shockingly, none of the currently known ecosystems appear to be very much like our own solar system. In fact, after some 25 years of research, it’s both disconcerting and surprising that we haven’t found a planetary system that dynamically, compositionally, or architecturally is really a solar system 2.0.

Twenty-five years ago, astronomers were only detecting the low-hanging planetary fruit, the wacky, wild hot Jupiters on short orbits around their parent stars because that’s what our ground-based Doppler spectroscopy surveys were most sensitive to finding. As a planet orbits its star on a short orbit, it can gravitationally perturb its parent star to such an extent that the effects can be measured in a given star’s radial velocity. That is, its motion towards or away from us along our line of sight. This causes what is known as a star’s center of gravity to be jerked around on the order of tens of meters per second. Our own Jupiter is even prone to such an effect on our Sun.

Thus, it was using this method that Mayor and Queloz were able to detect the gravitational effects of a planet half the mass of Jupiter on Pegasi 51, located some 50 light years distant in the Northern constellation of Pegasus.

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

More recently, Guillot’s focus has been largely on our own solar system using data gleaned from NASA’s orbital Juno mission to Jupiter, which has now been extended until 2025.  One of Juno’s goals was to put constraints on Jupiter’s deep composition.

“We’ve discovered that Jupiter and giant planets are not as simple as we thought,” said Guillot. “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 you get deeper into Jupiter, the gas changes into something more like a fluid, says Guillot. Even upon reaching the planet’s inner metallic hydrogen envelope, the composition is still that of a fluid, says Guillot. Near Jupiter’s center, the planet becomes ionized by two to four million Earth atmospheres of pressure, he says. This is why Jupiter has such high electrical conductivity which Guillot says is also likely the source of the planet’s extremely robust magnetic field. 

Guillot is part of a French team of astronomers who have been using the most isolated astronomical observatory on Earth —- an Antarctic ice station that is home to a 40-cm optical telescope —- to confirm many of the planetary detections that NASA’s TESS (Transiting Exoplanet Survey Satellite) has been making. 

The Antarctic Search for Transiting ExoPlanets (ASTEP) project is robotic but is part of the permanently staffed French-Italian Concordia base at Dome C, a 3,200-meter ice rise about a 1000 km from the geographic South Pole. Due to its three-month-long night during Antarctic winter, good weather conditions, and minimal atmospheric disturbance, ASTEP can make ground-based photometric observations that approach space-based quality.

We were able to observe the first circumbinary planet from the ground this year; a planet that is orbiting two stars like Tatooine (in Star Wars), says Guillot. We’re also looking for planets on long orbital periods transiting their stars, which are much harder to spot, he says.

Despite the fact that gas giant planets of any stripe are not likely to harbor any sort of life, their study continues to offer insight into planetary systems as a whole.

Giant planets had a huge impact on the architecture of our solar system and the flux of comets going into our inner solar system, says Guillot. Whether it was a positive or negative may depend on the system, he says.

Thus, something as mundane as having lunch at one of the world’s historic observatories —- windows onto the true quest to understand ourselves —- takes on fresh meaning. An act as quotidian as serving up a plate of hake fish in curry gives pause to marvel at how far this planet has come since it was being tossed around in our solar system’s protoplanetary disk.

Just how unique is our own solar system?

“For the moment, it’s still hard to tell,” said Guillot. 

Generally, the exoplanetary systems we’re discovering are quite different from ours and a show a huge diversity, he says. 

“I don’t think we have a twin of our own solar system in the sample yet; so, our solar system has to be pretty rare,” said Guillot.

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