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Simulated Earth-like planets show 68% stronger methane signal around older M dwarfs
A preprint led by UC Santa Cruz graduate student C. Evan Davis modelled Earth-like planets around M dwarfs of two ages. The star's ultraviolet output alone changed how much methane the atmospheres held by up to a factor of ten.
The Scientist · Science desk
What happened
- A preprint submitted to arXiv on Aug. 19 simulated Earth-like planets orbiting two types of M dwarf at ages from 650 million to 5 billion years. It is led by UC Santa Cruz astronomy graduate student C. Evan Davis.
- The methane signal an observer would measure from the older systems ran up to 68% stronger in the simulated data than from the young-star equivalents.
- The models used each star's usual, quiescent ultraviolet emission and left out short-lived flares.
- Writing in Live Science, University of Birmingham astrophysicist Alix Violet Freckelton argues that biosignature readings are being interpreted without accounting for the light of the host star itself.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- constraint Turning a methane band depth into a statement about biology now needs the host star's ultraviolet spectrum as an input, because on these models the same biological activity reads stronger around one star than another.
- decision Ranking candidates by how much starlight their transit blocks stops being sufficient. The easiest atmospheres to measure sit around the class of star whose ultraviolet history the simulations say does the most to the answer.
- exposure A first strong methane claim around an old, quiet M dwarf would land in exactly the regime where the modelled atmospheres accumulate the most gas for reasons that have nothing to do with life.
- contradiction The Live Science headline calls the search flawed. The work behind it tested simulated planets only, so the objection still has to face a real measurement.
The same simulated preindustrial atmospheres built up to 10 times more methane around the five-billion-year-old stars than around the 650-million-year-old ones [14]. Take both upper bounds as describing the same comparison, and the measurable signal moves about a sixth as far as the abundance does: a factor of 1.68 against a factor of 10 [17]. The Live Science account does not say whether the two maxima come from the same pair of models, so the ratio is indicative.
That weak response is a problem for interpretation. If band depth tracks abundance so loosely, a modest measured difference between two planets can sit on top of an order-of-magnitude difference in how much methane is actually there. The simulations attribute a difference that size to the host star [10]. The team varied stellar age and ultraviolet emission together while holding the planets comparable, so the change in the modelled spectra comes from the star's evolution [23].
M dwarfs are near the front of the queue for atmospheric work for a geometric reason. A planet crossing a small cool star blocks a relatively large fraction of its light, and that makes the atmosphere easier to detect [5]. They are also the most common type of star in the Milky Way [4].
The demonstration ran on entirely simulated planets [16], two kinds of M dwarf, and two atmospheric compositions. One was oxygen-rich preindustrial Earth; the other the Archean eon of roughly 4 billion to 2.5 billion years ago, when atmospheric oxygen was scarce [12]. Every atmosphere in it came out of a model. Published measurements stand where they were.
Live Science ran Freckelton's argument under the headline "The hunt for extraterrestrial life is fundamentally flawed. But there's a fix." [19] The case in the text is narrower. Methane, oxygen and ozone are the most studied biosignatures, and ozone is an indirect indicator of oxygen [6]. On Earth all three can be produced by geology, by chemistry, and by atmospheric reactions with sunlight [7]. A detection elsewhere, she writes, is "a clue, not proof of life" [8]. Freckelton works on how uncertainties in host stars affect what astronomers learn about their planets, and has analysed more than 2,000 of them [2][3].
Thousands of exoplanets have been found. Not one has yet produced convincing evidence of life [21].
What to watch
- Whether the Davis preprint clears peer review with the tenfold methane spread intact.
- Whether follow-up runs that add stellar flares move that spread up or down.
- Whether biosignature papers on M dwarf planets begin reporting the assumed host ultraviolet spectrum alongside retrieved abundances.