What’s An Average Exoplanet? ESA’s Upcoming ARIEL Mission Should Provide Answers

Five thousand detected extrasolar planets and counting, we still don’t have a standard model of the full range of planets circling other sunlike stars. Or so says University College London (UCL) astrophysicist Giovanna Tinetti, Principal Investigator for a consortium of several dozen institutions that are part of the European Space Agency’s (ESA) forthcoming 500 million euro ARIEL mission.

Using infrared and visible spectroscopy, the ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) mission should classify at least 1000 known exoplanets by the chemical makeup in their atmospheres. These days most exoplanet characterization efforts, both from ground and space, are focused on looking for an earthlike twin, an Earth 2.0.

But ARIEL is designed to offer the planetary science community a survey of all manner of extrasolar planets —- from terrestrial mass on up through gas giants.

Due to launch in 2029 to the gravitationally stable Earth-Sun L2 (Lagrangian point), ARIEL will take spectroscopic observations of a target planet as it transits around its parent star. Such transits enable an exoplanetary atmosphere to be characterized since it is backlit by its parent star. Thus, ARIEL will help planetary scientists determine whether the chemistry of a planet is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planet’s birth, and evolution.

“I’m interested in the big picture; how planets form and evolve in our galaxy,” Tinetti recently told me in her University College London (UCL) office. “All these planets will tell us a different story.”

Observations of these worlds will give insights into the early stages of planetary and atmospheric formation, and their subsequent evolution, in turn, contributing to the understanding of our own Solar System, says ESA.

Using its elliptical, one-meter class telescope, ARIEL will observe transiting gas giants, Neptunes, super-Earths, and earthsize planets around a range of host star types.

We will focus mainly on planets around very bright stars that are typically tens or in some cases hundreds of light years away, says Tinetti. That’s because the brighter the star, the easier it is to do these measurements, she says. And so will be able to better measurements even faster, says Tinetti.

Potentially, most of these planets will be warm and hot, says Tinetti.

Surprisingly, planetary theorists have made relatively little progress in the last twenty years in understanding how a planet’s host star may have influenced its formation and evolution.

“We have little idea whether the chemistry of a planet is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planet’s birth, and evolution,” Tinetti and co-authors wrote in a 2018 paper appearing in the journal Experimental Astronomy.

As for ARIEL’s planetary targets?

We want to make sure that we have a good statistical survey that includes different type planets around different types of stars, says Tinetti. We want to have an understanding how the atmospheric composition and characteristics change as a function of a wide variety of parameters, she says.

Depending on where the planets formed; either close to the star or much farther out, they might have captured different material in the protoplanetary disks, says Tinetti. And if we look at the atmospheric composition, we should be able to see the difference in terms of elemental abundances, she says.

ARIEL will provide us with the knowledge of the kind of exoplanetary atmospheric chemistry that we will be able to say for sure couldn’t harbor life, says Tinetti. But most importantly, she says, they will tell us what the normality is out there and give us a sort of standard model of non-habitable worlds.

Does Tinetti think Earth is rare?

“I don’t think we are rare,” she says. “But I’m interested in not just finding an Earth 2.0 but the cousins of Earth as well.”

As for finding life elsewhere?

I don’t want to be earthcentric and think that the only way to host life is to have an earthlike planet, says Tinetti. I want to keep my options open because I don’t think we have the full picture, she says.