This summer marks nearly three decades since the discovery of 51 Pegasi b, the first known extrasolar planet to orbit a Sun-like star. Today there are more than 5,000 known planetary systems orbiting sun-like stars, and it is now thought that half of all sun-like stars harbor planets.
The discoveries of exoplanets alone in the last decade — thanks in large part to the work of NASA’s now-defunct Kepler Space Telescope — are enough to boggle the mind. But astronomers are only now beginning to seriously characterize most of these planets. And that’s arguably where the focus should now be in this burgeoning field of exoplanetary science.
So, two years after Covid-19 hampered face-to-face meetings, one of the world’s premier exoplanetary science conferences — Extrasolar Planets IV (Exo4) — has just wrapped up in Las Vegas. Last week, I was able to catch up with Exo4’s organization president, Jason Steffen, to chew on some of the field’s key issues.
At the top of my list was simply why, after decades of searching with both ground and space telescopes, we haven’t found a true exo-Earth.
We know of Earth-sized planets that are near the habitable zone, Steffen, an astrophysicist at the University of Nevada Las Vegas, told me. But he says that in terms of understanding the properties of their atmospheres; the nature of liquid water in the atmosphere or on their surfaces, we are still a generation away from telescopes that can give us those kinds of measurements.
When do we actually get spectra of an exo-Earth?
2050 is a gamble, 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 ours,” Steffen said.
We have a relatively good grasp of how our solar system formed and evolved, but exoplanetary science says these are all the other things that haven’t happened in our solar system spawning different kinds of planets, he says.
As for the synergy between solar system science and exoplanetary science?
Planetary scientists who focus on bodies in our own solar system have an abundance of wealth, Steffen says. Mars researchers have had the luxury of taking samples from the surface there and conducting in situ analyzes that may indicate an abundance of dozens of chemical compounds. Solar system scientists also have access to the world’s best ground-based spectrometers that can identify dozens of chemical species on bodies across our solar system — from Mercury to Pluto.
But right now, extrasolar planet researchers are lucky if they can detect hydrogen in an exoplanet’s atmosphere, Steffen says. He notes, however, that there is an area where there is more fair competition. That is in extrasolar dynamic measurements of the movements of a particular planet. And how the movements of one planet affect the movements and movements of other planets within the same system.
We can understand the orbital properties of exoplanetary systems and compare them with the orbital properties of planets in our solar system, Steffen says.
One of the more interesting presentations at the Exo4 conference involved identifying putative planetary material accumulated on dying stellar remnants known as white dwarfs.
White dwarf stars are super-dense, and if you dumped something on top of a white dwarf, it would only remain visible on the surface for a few thousand years before it all sinks into the interior, Steffen says.
So if you observe something that only has a lifespan of a thousand years on a star that’s been there for a billion years, you know it must have been a recent influx on the surface of a white dwarf, Steffen says. 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 abundance of nickel, iron and sodium, Steffen says.
Could this material be from planets destroyed by the stellar endgame of the system itself?
It is not clear where that material comes from; Whether it’s planets destroyed in the star’s red giant phase, or before the planet was engulfed by the dying red giant, Steffen says.
The other big discussion on Exo4 was evidence for the existence of a third terrestrial mass planet orbiting our closest stellar neighbor, Proxima Centauri. Just 4.2 light-years away, Proxima Centauri is a faint red dwarf that is literally the next star.
The evidence that there is a third terrestrial planet seems compelling, Steffen says. Whether it’s habitable seems a bit far-fetched, but the fact that we’re observing this around the nearest star just shows how common planet formation really is, he says.
Is this a numbers game? Should we go out and find most planets or study them in detail?
We haven’t done detailed studies of even 10 percent of the planets discovered by Kepler, Steffen says. While it’s valuable to find more planets, it’s also valuable to understand the planets we’ve discovered, he says.
Steffen says the Webb Space Telescope and the next generation of extremely large ground-based telescopes are a way to characterize atmospheres from many of the planets we’ve found. Observations spanning a longer period of time also provide insight into the systems in which those planets reside, he says.
But exoplanetary science still lacks the kind of funding it needs to enable more high-risk, high-reward initiatives, Steffen says.
“Everything is so competitive that the vast majority of proposals are rejected,” says Steffen. “The current financing situation [makes] the discipline is too risk averse.”
A 30 percent pass rate for grant proposals would be much healthier than the less than 10 percent pass rate we see now, he says.
“Science would progress faster if there was enough room for more studies that don’t come true,” Steffen says.