If astrobiologists are ever going to detect life beyond our solar system, it will arguably be done via spectroscopy of a planetary atmosphere as it passes in front of its parent star. Problem is, even nearby earth-like planets are extremely difficult to detect.
So, most of these extrasolar gas giants thus far detected are the size of Neptune or larger. Unfortunately, these gaseous exo-giants are hardly harbingers of life as we know it. But they are a necessary stop on the long torturous road to detecting extrasolar life.
Three decades ago, Swiss astronomers Michel Mayor and Didiez Queloz of Geneva Observatory, detected the first planet orbiting another sunlike star. Roughly half the size of our own Jupiter, they found it on an incredibly and unexpectedly short 4.2-day orbit around the solar type star 51 Pegasi.
Located some 51 light years away in the northern constellation of Pegasus, about the only thing the Swiss duo could determine about the planet at the time was its mass and orbit. But given the planet’s proximity to its star, they determined it had to be a searingly hot gas giant planet, the prototype for a new class of planet now known as hot Jupiters.
Beyond Detection To Characterization
We have moved from pure detection to characterization which has changed our understanding of atmospheric chemistry and physics, astrophysicist Christiane Helling, director of the Institute for Space Science at the Austrian Academy of Sciences, told me in Graz. We are studying these planets’ chemical compositions, their thermodynamics and their wind fields, she said. And it has now been proven observationally that these planets have very complex atmospheric chemistry, said Helling.
Extended Atmospheres
These gas giants are the easiest to observe; they have wonderfully extended atmospheres, so we can get information from each part of these gas giants’ atmospheres, said Helling. This helps us to either prove that our models are correct, or if not, to improve them, she said. These are the first steps before we really can understand extraterrestrial rocky planets, said Helling.
Future Of Exo-Earth Characterization
The computer modeling tools we use on the spectra and atmospheres of gas giant planets today are the same kind of tools we will use to interpret the spectra of potential exo-Earths in the future, said Fortney.
Space telescopes like the James Webb have made a huge difference.
We can detect and measure the abundances of molecules like water, methane, carbon dioxide, ammonia, and sulfur dioxide, along with the abundances of at least 10 atoms in very hot planets, like sodium, potassium and iron, planetary astrophysicist Jonathan Fortney at the University of California in Santa Cruz, told me via email.
Astronomers are now able to take inventory of these atoms and molecules and compare their compositions both from planet to planet as well as to their parent stars.
We can do that for ten to twenty planets now, but I think we’ll be able to do that for a couple of hundred planets in a few years’ time, said Fortney.
Similar gas giant planets can have vastly different atmospheric compositions with disparate cloud or haze properties. Located some 869 light years away, WASP-19b is one of the most extreme examples of a hot Jupiter, it’s what is known as an ultra-hot Jupiter that orbits a sunlike star in constellation of Vela every 0.79 days. But it’s only one of millions out there that run the gamut in terms of their proximity to their parent stars, their chemical makeup and their size.
Thus, such work requires large sample sizes.
Taking atmospheric measurements of a single hot Jupiter in its current state and extrapolating back several billion years to its formation requires making a lot of assumptions, Kevin Stevenson, an astrophysicist at Johns Hopkins Applied Physics Lab in Laurel, Maryland, told me via email. But by characterizing hundreds of hot Jupiters that span a range of ages and metallicities can reveal population-level trends, he said. From these trends, we will soon be able to infer distinct formation and evolution scenarios for different groups of exoplanets, said Stevenson.
These extrasolar gas giant planets run the gamut and are a diverse and complex group. But there’s hope.
Thanks to the James Webb Space Telescope, when it comes to hot Jupiter atmospheres, I believe that we’ll solve many of today’s outstanding mysteries within the next five years, said Stevenson.
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