Looking for Life? Try Around K Dwarfs

Signs of life in planetary atmospheres are hard to spot! A new study suggests that the best strategy for discovering them may be to look at planets orbiting K-dwarf stars.

The Hunt for Fingerprints

Is there life beyond Earth? This remains one of the most profound scientific questions astronomers are currently poised to address, and development of ever more powerful telescopes continues to bring us closer to an answer.

One way we can hope to use these telescopes to identify the presence of life on distant exoplanets is by detecting atmospheric biosignatures. Left alone, a planet’s atmosphere ought to be in chemical equilibrium. But when life is present, the atmosphere accumulates excess gases produced by the life — telltale fingerprints that we can hope to spot.

Signatures in Methane

A good example of these fingerprints is the simultaneous presence of oxygen and methane in a planet’s atmosphere — something that shouldn’t occur if life isn’t there. The hunt for this biosignature is complicated by the fact that methane in the presence of oxygen is destroyed via chemical reactions driven by stellar light; if too much of the methane is removed by these photochemical reactions, we won’t be able to detect it.

There’s hope, though: some planets may be more likely to maintain life-produced methane in their atmospheres than others. Stellar light in the 200–350 nm range triggers this reaction — so the less light a planet’s host star produces in this range, the longer methane can survive in the planet’s atmosphere. This means that the type of host star matters: G dwarfs like the Sun will destroy the methane in their planets’ atmospheres faster than smaller and cooler M dwarfs.

Unfortunately, M dwarfs have other complications hindering the potential for life — including high levels of stellar activity that drive atmospheric loss from their planets. For this reason, scientist Giada Arney (NASA Goddard SFC and NASA NExSS Virtual Planetary Laboratory) has explored the advantages of a different type of star: K dwarfs.

The K-Dwarf Advantage

K dwarfs fall between G and M dwarfs in size and temperature, and they are more abundant than G dwarfs. Dimmer than G dwarfs, K dwarfs provide better planet-to-star contrast ratios that make it easier to observe potential habitable worlds. And they are are less active than M dwarfs, providing a more hospitable environment for their planets.

In addition to these benefits, K dwarfs also produce less radiation in the 200–350 nm range than G dwarfs do. By using a one-dimensional photochemical climate model to simulate a variety of planetary atmospheres, Arney demonstrates that a planet orbiting a K6V star can support roughly an order of magnitude more methane in the presence of oxygen relative to an equivalent planet around a G2V dwarf like the Sun.

But is this enough to produce signatures we can soon detect? Arney uses synthesized spectra to show that, with the technologies proposed for potential future missions like LUVOIR or HabEx, we have a decent chance of spotting the simultaneous methane and oxygen signatures in planets orbiting nearby mid-late K-dwarf stars. Thus the “K-dwarf advantage” gives us a great list of promising targets for the next major space missions!

Citation

“The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets,” Giada N. Arney 2019 ApJL 873 L7. doi:10.3847/2041-8213/ab0651