Space & Cosmos

Exoplanets may have hidden atmospheres

Diagram of the tidally locked planet TRAPPIST-1f. Courtesy of NASA/JPL-Caltech

Most of the exoplanets we’ve discovered orbit red dwarf stars. This isn’t because red dwarfs are special, but simply because they’re common. About 75% of the stars in the Milky Way are red dwarfs, so they’re likely to be the most abundant. This also means that most habitable worlds orbit these small, cool stars, which has important implications for our search for life.

First, any potentially habitable red dwarf world needs to orbit close to its star in order to be warm enough for liquid water, etc. to exist. The TRAPPIST-1 system, which I talked about yesterday, is a good example of this. The three potentially habitable planets in this system orbit a fraction of the distance between Mercury and the Sun, which means they are at risk of stellar flares, etc., but also means they are almost certainly tidally locked.

Tidal locking occurs when a planet or moon is so close to a companion star that tidal forces synchronize its rotation with its orbital motion. When a planet is tidally locked, one side always faces its star and the other side is permanently in darkness.

As you might expect, this means that the warm side will bake while the other side freezes. This is true unless the planet has a favorable atmosphere; if it has a water-rich, Earth-like atmosphere, heat can transfer between the day and night sides. Such a world would have weird weather, but a tidally locked world could have roughly uniform temperatures on its day and night sides and be habitable.

Exoplanets may have hidden atmospheres

Clouds make the planet appear airless. Credit: Powell et al.

Observing the atmosphere of a tidally locked planet is difficult, but astronomers have a trick to tell if an atmosphere exists: Instead of capturing the atmosphere’s spectrum, they simply measure the surface temperature on the far side of the planet.

So we look at the star as the planet moves in front of it to determine the temperature of the dark side, then look at the star again as the planet moves behind it to determine the temperature of the bright side. If the temperatures of the dark side and bright side are dramatically different, the star must have no atmosphere. Simple. But new research shows that’s not necessarily true.

This paper is published on the arXiv preprint server.

In the paper, the authors argue that clouds on the world’s dark side could distort the data. To demonstrate this, the authors imagine a tidally locked world with a thick atmosphere. According to their model, the atmosphere moderates temperatures across the planet’s globe, with the dayside being only a few tens of degrees warmer than the dark side. This is similar to the extreme changes between day and night in arid regions of Earth. Although the temperature changes are mild, they are enough to form thick clouds on the dark side.

In this scenario, the dayside would be mostly cloud-free, and we would measure warm temperatures on the planet’s surface. But if the dark side were covered in clouds, we would measure temperatures in the upper cloud layers, which would be much cooler. Thus, even though the planet’s surface temperature is nearly uniform, it would appear to have extreme temperature variations, like an atmosphere-less world. The authors consider how JWST’s observations might distinguish between planets with clouds and planets without atmospheres, but it’s clear that one simple trick in the search for habitable planets is not so simple.

Further information: Diana Powell et al. “Nightside clouds on a tidally locked terrestrial planet mimic an atmosphere-less scenario.” arXiv (2024). DOI: 10.48550/arxiv.2409.07542

Journal information: arXiv

Courtesy of Universe Today

Source: Exoplanets May Have Hidden Atmospheres (September 20, 2024) Retrieved September 21, 2024 from https://phys.org/news/2024-09-exoplanets-atmospheres.html

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