Since the 1970s, we've been searching for life outside Earth in our universe. Astronomers have discovered evidence of life in prebiotic chemistry and biological compounds throughout our solar system and beyond. Transmission spectroscopy — detecting chemical signatures from light passing through exoplanet atmospheres — has been used with some success, but observing the surfaces directly may prove to be a better solution. A new study looks at how Earth might appear as an exoplanet, simulating spectra generated with the Planetary Spectrum Generato (PSG) and retrieved with the Exoplanetary Reflected Light Retrievel (ExoReLℜ).
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Astro Brief is a podcast collaboration between KSMU, the Missouri Space Grant, and MSU's Department of Physics, Astronomy and Materials Science. Hosted by Dr. Mike Reed, Astro Brief focuses on astronomical events, the field of astronomy, and astronomy-related guests. It airs Thursdays at 9:45 am on KSMU.
Transcript
We've spent a lot of episodes talking about life — It is certainly one of the biggest unanswered questions of the human experience.
Are we alone in the universe?
It's a question that humans have wondered about ever since we understood that the sky wasn't just a cover over our planet, but a whole universe of stars and planets.
Beginning in earnest with the 1970s Viking landers on Mars, we are working to obtain evidence of whether or not life exists — either within our own solar system or out among those stars. And amazingly, it's a question that will likely have at least a statistical answer during my lifetime.
We've discussed prebiotic chemistry splattered all across our solar system and presumably many others, along with somewhat more direct chemical evidence on Mars — namely biological carbon and fatty acid compounds. Within another decade, better studies of more solar system objects will increase our understanding, either for or against the possibility of life within our solar system. We are also getting better at looking for life on planets outside of our solar system.
In a previous episode, we reported the possible discovery of dimethylsulfide — a molecule which in our atmosphere is produced by life. That discovery remains somewhat controversial, but just the fact that we got a spectrum of another planet's atmosphere is pretty amazing. That spectrum was obtained by JWST and it has some capability to do the job we're after, but it's limited. That possible measurement of dimethylsulfide wasn't an earth-sized planet, but rather a gassy mini-neptune.
JWST has tried to observe atmospheres of some super-earth-sized planets, but it has yet to achieve convincing results. We're going to talk about a new technique recently published by Dr. Zachary Burr of the Jet Propulsion Laboratory and collaborators — published in the Astrophysical Journal — and a summary of Dr. Burr's and related works from an American Astronomical Society astrobite by Kaz Gary.
Let's begin with the review of the most common technique of looking for life on exoplanets, transmission spectroscopy.
The goal is to look at a planet's atmosphere as it passes in front of its star and some of that starlight goes through the planet's atmosphere. In our own atmosphere, biosignature gases would include: oxygen and ozone — maintained by our plants, methane — produced by gut bacteria, nitrous oxide — produced by non-oxygen-using bacteria, and carbon dioxide exhaled by us and other animals. Other trace gases include ammonia from fungi, isoprene from decaying foliage, and sulfur gases from cyanobacteria. All of those would be atmospheric gases which could be observed on exoplanets once we have the technological capability, which — as previously mentioned — we currently do not. But there's the Roman infrared telescope set to launch next year and the upcoming Habitable World's Observatory still in the design phase.
The new study by Dr. Burr collaborators shows that looking at the planet itself — rather than transmission spectroscopy — is a better way to go.
We have discussed this before, but essentially when a planet is on the far side of its star, we can see the planet's surface being illuminated by the star. Now we don't expect to see the surface like the recent pictures taken during the Artemis-2 mission of our own planet from space. At the distance of exoplanets they are all dots — single pixels of light — but that light would include ingredients from the planet. Dr. Burr's study examined what spectra of that single pixel would look like for Earth, and most importantly comparing what would be seen in optical versus infrared light.
That's because we have something called the vegetation red edge.
Our green plants use chlorophyll to absorb sunlight, but that mainly works in the optical and reflects in the infrared. Beginning around 670-750 nanometers, trees reflect about 50% of the sun's light, and coral an amazing 80%. So that type of life is much brighter in the infrared than in the optical. But the Earth is spinning and has clouds, which produces changing quantities of plant life visible from space.
Dr. Burr's group looked at nine spectra of Earth taken during different times, as well as a spectrum summed over one full rotation. In all of the spectra, the red edge caused by plants was still detectable and — excluding the plant's contributions — created non-physical results on the Earth's size and mass. Their results indicate that direct surface observations — rather than transmission spectroscopy — could be easier, and that seeing red could mean the planet is in the green.