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What is an average exoplanet? ESA’s upcoming ARIEL mission should provide answers

Five thousand detected planets beyond our solar system and we still don’t have a standard model of the full range of planets orbiting other sun-like stars. So says Giovanna Tinetti, an astrophysicist at University College London (UCL), principal investigator of a consortium of several dozen institutions that are part of the European Space Agency’s (ESA) forthcoming EUR 500 million ARIEL mission.

Using infrared and visible spectroscopy, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) mission is to classify at least 1,000 known exoplanets based on the chemical composition in their atmospheres. Today, most efforts to characterize exoplanets, both from the ground and from space, have focused on the search for an Earth-like twin, an Earth 2.0.

But ARIEL is designed to provide the planetary science community with an overview of all kinds of exoplanets – from terrestrial masses to gas giants.

Due to launch in 2029 to the gravitationally stable Earth-Sun L2 (Lagrangian point), ARIEL will make spectroscopic observations of a target planet as it orbits its parent star. Such transitions make it possible to characterize an exoplanetary atmosphere as it is illuminated by its parent star. Thus, ARIEL will help planetary scientists determine whether a planet’s chemistry is related 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 office at University College London (UCL). “All these planets will tell us a different story.”

Observations of these worlds will provide insight into the early stages of planetary and atmospheric formation, and their subsequent evolution, which in turn contribute to understanding our own solar system, says ESA.

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

We will mainly focus 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 make these measurements, she says. And so we can arrive at better measurements even faster, says Tinetti.

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

Surprisingly, over the past two decades, planetary theorists have made relatively little progress in understanding how a planet’s parent star may have influenced its formation and evolution.

“We have no idea whether a planet’s chemistry is related 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 to appear. in the journal Experimental Astronomy.

As for ARIEL’s planetary goals?

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

Depending on where the planets formed; close to the star or much further away, they could have trapped other material in the protoplanetary disks, says Tinetti. And if we look at the atmospheric composition, we should be able to tell the difference in terms of elemental abundance, she says.

ARIEL will provide us with the knowledge of the kind of exoplanetary atmospheric chemistry that we can safely say cannot support life, says Tinetti. But most importantly, she says, they’ll tell us what’s normal out there and give us a kind of standard model of non-habitable worlds.

Does Tinetti think the Earth is rare?

“I don’t think we’re rare,” she says. “But I’m interested not only in finding an Earth 2.0, but also in Earth’s cousins.”

As for finding a life elsewhere?

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

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