Signs of life? Why astronomers are excited about carbon dioxide and methane in otherworldly atmospheres

Are we alone? This question is almost as old as humanity itself. Today, this astronomy question focuses on the search for life beyond our planet. Are we, as a species and a planet, alone? Or is there life elsewhere?

Often the question inspires visions of strange, green versions of humans. However, there’s more to life than just us: animals, fish, plants and even bacteria are all things we look for signs of in space.

One thing about life on Earth is that it leaves its mark on the chemistry of the atmosphere. So such tracks, visible from afar, are what we look for when hunting for aliens.

Scientists in the United Kingdom and the United States have just reported some very interesting chemical traces in the atmosphere of a planet called K2-18b, which is about 124 light-years from Earth. In particular, they may have detected a substance which on Earth is only produced by living things.

Meet exoplanet K2-18b

K2-18b is an interesting exoplanet, a planet orbiting another star. It is a type of planet called a sub-Neptunian, discovered in 2015 by the Kepler Space Telescope’s K2 mission. As you probably guessed, these are smaller than Neptune in our solar system.

The planet is about 8.5 times more massive than Earth and orbits a type of star called a red dwarf that is much cooler than the Sun. But K2-18b orbits much closer to its star than Neptune, within the so-called habitable zone. This is an area that is neither too hot nor too cold, and where liquid water can exist (rather than freezing to ice or boiling to steam).

Earth is a so-called rocky planet (for obvious reasons), but the planets below Neptune are gas planets with much larger atmospheres containing a lot of hydrogen and helium. Your vibe may also include other elements.

Now you can feel the excitement surrounding K2-18b.

How to fingerprint an atmosphere

The planet was first discovered by the Kepler space telescope, which was monitoring distant stars and hoping that planets would pass in front of them. When a planet passes between us and a star, it dims momentarily, which tells us that there is a planet there.

By measuring the magnitude of the decrease in brightness, the time it takes the planet to pass in front of the star, and how often this happens, we can determine the size and orbit of the planet. This technique is useful in finding planets, but it doesn’t tell us about their atmospheres, which is important information for understanding whether they harbor life or are habitable.

NASA’s James Webb Space Telescope, a large space telescope launched in late 2021, has now observed and measured the exoplanet’s atmosphere.

The telescope achieves this by measuring the color of light so finely that it can detect traces of specific atoms and molecules. This process, called spectroscopy, is intended to measure traces of elements.

Each element and molecule has its own color signature. If you can look at the color signature, you can do a bit of detective work, and work out what elements or compounds are in the planet.

While the planet does not have its own light, astronomers waited for when K2-18b passed in front of its star, and measured the starlight as it went through the planet’s atmosphere, allowing the team to detect fingerprints of substances in the atmosphere.

Alien marine farts?

New research finds lots of carbon dioxide and methane. This is interesting because it is more similar to what we find on Earth, Mars and Venus in our solar system, than Neptune.

However, small amounts of dimethyl sulfide were also found. Dimethyl sulfide is an interesting molecule, composed of carbon, hydrogen and sulfur.

On Earth there is generally a slightly unpleasant smell.But it is also associated with life.

The only process we know of that produces dimethyl sulfide on our planet is life. In particular, marine life and plankton emit it in the form of flatulence.

So scientists are excited about the potential idea of ​​alien sea farts. If it’s real. And associated with life.

The search continues

While on Earth, dimethyl sulfide is linked to life, on other planets it may somehow be related to geological or chemical processes.

After all, K2-18b is something like Neptune—a planet we do not really know a lot about. Just last month, researchers discovered that clouds on Neptune are strongly linked to the sun’s 11-year cycle of activity. We have a lot to learn about planets and their atmospheres.

Also, the measurement of dimethyl sulfide is very subtle—not nearly as strong as the carbon dioxide and methane. This means more detailed measurements, to improve the strength of the signal, are required.

Other telescopes may need to join the effort. Instruments on the Very Large Telescope in Chile are able to measure the atmospheres of planets around other stars—as is a new instrument called Veloce on the Anglo Australian Telescope at Siding Spring Observatory in Australia.

And new space telescopes, like Europe’s PLATO which is under construction, will also help us get a better look at alien atmospheres.

So while the signs of dimethyl sulfide on K2-18b may not be linked to life, they are still an exciting prospect. There is plenty more to explore.