Astronomers Should Shift Focus To Understanding Exoplanets We’ve Already Discovered, Says Researcher

This summer marks nearly three decades since the discovery of 51 Pegasus b, the first known extrasolar planet to circle a sunlike star. Today, there are more than 5000 known planetary systems circling sunlike stars and as many as half of all sunlike stars are now thought to harbor planets.

Exoplanet discoveries in the last decade alone — in large part due to the work of NASA’s now defunct Kepler Space Telescope — are enough to boggle the mind. But astronomers are just now beginning to characterize most of these planets in earnest. And arguably that’s where the focus in this burgeoning field of exoplanetary science should now be.

Thus, two years after Covid-19 stymied in-person meetings, one of the world’s major exoplanetary science conferences — Extrasolar Planets IV (Exo4) — has just wrapped up in Las Vegas. This past week, I was able to catch up with Exo4’s organizing chair, Jason Steffen, to chew over some of the field’s major issues.

At the top of my list was simply why after decades of looking with both ground- and space-based telescopes, we’ve yet to find a veritable exo-earth.

We know of Earth-sized planets that are near the habitable zone, Steffen, an astrophysicist at the University of Nevada in Las Vegas, told me. But he says that in terms of understanding the properties of their atmospheres; the nature of any liquid water in the atmosphere or on their surfaces, we are still a generation away from telescopes that can give us those types of measurements.

When will we actually start getting spectra from an exo-earth?

2050 is a guess, says Steffen.

What does our study of exoplanets tell us about our own solar system?

“That you can have solar systems that look very different from our own,” said Steffen.

We have a relatively good handle on how our solar system formed and evolved, but exoplanetary science says here’s all the other things that didn’t happen in our solar system that produces different kinds of planets, he says.

As for the synergy between solar system science and exoplanetary science?

Planetary scientists who focus on bodies within our own solar system have an abundance of riches, says Steffen. Mars researchers have had the luxury of taking samples of the surface there and doing in situ analysis which can indicate abundances of dozens of chemical compounds. Solar system scientists also have access to the world’s finest ground-based spectrometers which can identify dozens of chemical species on bodies throughout our solar system —- from Mercury to Pluto.

But at this point, extrasolar planet researchers are lucky if they can detect hydrogen in the atmosphere of an exoplanet, says Steffen. However, he notes that there’s one area where there’s more of a fair competition. That is in extrasolar dynamical measurements of a given planet’s movements. And how the motions of one planet affects the motions and movements of other planets within the same system.

We can understand the orbital properties of exoplanetary systems and compare those with the orbital properties of planets in our solar system, says Steffen.

One of the more interesting presentations at the Exo4 conference involved identifying putative planetary material accreted onto dying stellar remnants known as white dwarfs.

White dwarf stars are superdense and if you dumped something onto a white dwarf it would only stay visible on the surface for a few thousand years before it would all sink into the interior, says Steffen.

So, if you observe something that only has a one-thousand-year lifetime on a star that’s been there for a billion years, that tells you it must have been a recent influx onto the surface of a white dwarf, says Steffen. That must be leftover planetary stuff, he says. This is the only method I know of where you can measure the composition of the planet forming material; that is, the nickel, iron and sodium abundance, says Steffen.

Would this material have originated from planets that were destroyed by the stellar endgame of the system itself?

It’s not clear where that material originates; whether from planets that have been destroyed in the star’s red giant phase, or before the planet was engulfed by the dying red giant, says Steffen.

The other big discussion at Exo4 was evidence for the existence of a third terrestrial mass planet circling our nearest stellar neighbor, Proxima Centauri. Located only 4.2 light years away, Proxima Centauri is a faint red dwarf that is literally the next star over.

The evidence that there’s a third terrestrial planet seems convincing, says Steffen. Whether it’s habitable seems a bit of a stretch but the fact that we observe this around the nearest star to us just indicates how common planet formation actually is, he says.

Is this a numbers game? Should we be out to find the most planets or to study them in detail?

We haven’t done detailed studies of even 10 percent of the planets that have been discovered by Kepler, says Steffen. While there’s value in finding more planets, there’s also value in understanding the planets that we’ve discovered, he says.

Steffen says that the Webb Space Telescope and the next generation of ground-based extremely large telescopes are one way to characterize atmospheres of many of the planets that we’ve found. Observations that span a longer time period also add insights into the systems where those planets reside, he says.

But exoplanetary science still lacks the kind of funding it needs to enable more high risk, high reward initiatives possible, says Steffen.

“Everything is competitive to the point where the overwhelming majority of proposals get rejected,” said Steffen. “The current funding situation [makes] the discipline too risk averse.”

A 30 percent success rate for grant proposals would be much healthier than the less than 10 percent success rate that we see now, he says.

“Science would advance faster if there was enough room for more studies that don’t pan out,” said Steffen.


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