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

Five thousand exoplanets detected and counting, we still don’t have a standard model of the full range of planets that surround other sun-like 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) upcoming €500 million ARIEL mission.

Using infrared and visible spectroscopy, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) mission is expected to classify at least 1000 known exoplanets based on the chemical composition of their atmospheres. These days most exoplanet characterization efforts, both from the ground and from space, focus on the search for an earth-like twin, an Earth 2.0.

But ARIEL is designed to offer the planetary science community an investigation of all types of extrasolar planets, from land masses up 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 transits around its parent star. Such transits allow an exoplanetary atmosphere to be seen as it is backlit by its parent star. Thus, ARIEL will help planetary scientists determine whether a planet’s chemistry is related to its forming 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 told me recently in her University College London (UCL) office. “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, in turn contributing to our understanding of our Solar System, says ESA.

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

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

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

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

“We have no idea whether a planet’s chemistry is related to its forming environment, or whether the type of host star drives the physics and chemistry of the planet’s birth and evolution,” wrote Tinetti and co-authors in a paper in the 2018 appeared in the diary Experimental astronomy.

As for ARIEL’s planetary goals?

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

Depending on where the planets formed; close to the star or much further away, they may have captured different material in protoplanetary disks, Tinetti says. And if we look at atmospheric composition, we should be able to see the difference in terms of elemental abundance, he says.

ARIEL will provide us with knowledge of the kind of exoplanetary atmospheric chemistry that we can say with certainty could not support life, Tinetti says. But more importantly, he says, they will tell us what normality is out there and give us a kind of standard model of uninhabitable worlds.

Does Tinetti think the Earth is strange?

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

As for finding life elsewhere?

I don’t want to be terracentric and think the only way to harbor life is to have an earth-like planet, Tinetti says. I want to keep my options open because I don’t think we have the full picture, he says.


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