Life's Chemistry Needs Just Enough Starlight, And These Exoplanets Have It

An artst's rendition of Kepler-452b, which gets just enough light to be able to feature the chemical inventory needed to produce RNA. NASA Ames/JPL-Caltech/T. Pyle

Robin Andrews 02 Aug 2018, 20:30

We don’t quite know how life on Earth began, but we know that it required a series of somewhat complex molecules. These so-called “building blocks of life” have been found elsewhere in the Solar System, but what conditions would you need to get these precursors to the DNA and RNA required for life to appear in the first place?

That was the question on the collective mind of a team led by Cambridge University and the Medical Research Council Laboratory of Molecular Biology. By using some rather nifty chemistry experiments, they’ve suggested that there are several exoplanets out there where such building blocks can emerge.

The Science Advances study explains that these key large organic compounds likely arose through the interaction of smaller compounds in the presence of ultraviolet light streaking from a local star.

Unlike other theories as to how such compounds could emerge, these photochemical reactions are documented as being chemically plausible. They could potentially lead to RNA, which is particularly important: it doesn’t just store information, but also helps other molecules to react with each other, making its chemical precursors arguably more vital than DNA’s.

Still, it wasn’t clear how much UV light was needed to make said building blocks in the first place. By using common sulfur-bearing compounds, and seeing how they react in the presence and absence of light – “light” and “dark” chemistry, respectively – in a lab, they came up with a range of estimates.

SO3 compounds, for example, can readily turn into the “prebiotic inventory” near K-type stars, which are a little smaller and cooler than our own (a G-type star). Even cooler stars could trigger this photochemistry, so long as they were more hyperactive than usual.

Their results suggest that it doesn’t take that much light to initiate this key step in forming RNA. Applying their results to a catalogue of known exoplanets, the team found that the cosmos was rich with possibilities.

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Just one planet so far sits within the habitable and abiogenesis zones: Kepler-452b. University of Cambridge 

From several of the TRAPPIST worlds to others discovered by the Kepler space telescope, there were plenty of rocky spheres with the requisite UV levels. Forget planets: even large moons around gas giants may be primed for life.

These worlds all lie within the abiogenesis zone, denoting places from which the prebiotic inventory, and ultimately life, could arise from such photochemistry. Although it’s estimated that plenty of exoplanets within this zone also overlap with their stars' habitable zones – where liquid water can exist –, the team found just one world that we know definitely does: Kepler-452b, a slightly older planetary cousin to Earth in some ways.

A key problem with this study though, as with all others like it, is that we only have one model for the emergence and evolution of life: Earth. It hasn’t been detected anywhere else, even if the chemical building blocks of it has.

Statistically, life on other worlds is likely to exist, but it very well may not follow the same biological model as ours. What if there are other ways, outside of RNA and DNA and its chemical precursors, to give rise to it?

Starlight obviously giveth, but it can taketh away too: A world that might be suitable and may even contain the prebiotic library thanks to those kick-starting UV rays may also find that it’s regularly bombarded by huge stellar flares too, which could interrupt the key chemical reactions.

The team are clearly aware of this, but their study is delightfully infused with rational optimism regardless. “It turns out that stellar activity is not always bad for life but may, in fact, be the only pathway to starting life on planets around ultracool stars,” they explain.

Powerful solar flares could prove to be a problem. NASA

It’s hardly a perfect study. As pointed out by Space.com, the experiments didn’t manage to produce RNA precursors under early Earth-like conditions, which seems odd. Other researchers worry that the chemicals used in the mix are outdated, oversimplified mixtures.

If their chemistry is correct, though, we may one day find biosignatures out there not near a Sun-like star but around a far chillier furnace. That won’t just mean we’re no longer alone in the universe; it also reveals that chemistry doesn’t just find a way, but multiple ways, to transform into life.

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