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Title: Carbon Cycle Imbalances on Arid Terrestrial Planets with Implications for Venus
Authors: Haskelle T. White-Gianella and Joshua Krissansen-Totton
Authors’ Institution: University of Washington
Status: Published in PSJ

The Carbon-Cycle Thermostat

When discussing the habitability of planets, we usually focus on the habitable zone, the region around a star where a planet can host a temperature suitable for supporting liquid water on its surface. Yet, being in the right place doesn’t necessarily guarantee that a planet will be habitable.

A planet also needs the right atmospheric composition to sustain the necessary temperatures for hosting liquid water. A key mechanism found on Earth for maintaining this composition is the geologic carbon cycle, which acts as a built-in climate control system. This cycle begins with volcanic eruptions releasing carbon dioxide (CO₂₎, which then dissolves in rainwater and forms a weak acid that weathers rocks on continents. The weathering products wash into oceans, where they form carbonate rocks, locking carbon away. Eventually, plate tectonics recycles some of that carbon back to volcanoes. The weathering process works faster when the planet is warmer, thus regulating the amount of CO₂ and stabilizing the climate over long periods of time (but it should be noted that it can take up to a few hundred thousand years to rebalance this slow carbon cycle through the weathering process).

An important caveat is that the weathering process requires sufficient liquid water on the planet’s surface, so what happens in the case for planets with shallower oceans? Low-mass M dwarfs, the most common type of star found in the galaxy, are expected to host less-massive disks and therefore lower water inventories for forming planets. If more “dry” planets are indeed a more likely outcome of planet formation, we’d want to know how their arid conditions influence their long-term evolution and habitability.

Creating Carbon-Tracking Models

The authors built a model that tracks how water and CO₂ move between a planet’s interior, oceans, and atmosphere over 4.5 billion years. The model notably includes the following:

• A sophisticated weathering model that accounts for how runoff can limit rock weathering, improving upon previous models that only included a temperature dependence on weathering
• Wind-driven evaporation limits that influence the amount of water evaporation in addition to sunlight-driven evaporation
• Multiple deep water cycle parameterizations that explore how water moves between the interior and the surface

The model tests for four initial surface water inventories (0.1%, 1%, 10%, and 100% of Earth’s oceans) and then simulates how the surface water, atmosphere, and climate evolve over time.

A Critical Water Threshold

The models reveal that planets require at least 20–50% of Earth’s ocean mass to maintain a balanced carbon cycle. Below this threshold, the carbon cycle becomes unbalanced, leading to devastating consequences for habitability (Figure 1).

For the more arid planets, models reveal a concerning runaway process. Limited surface water reduces precipitation and slows rock weathering. Weathering cannot keep up with volcanic CO₂ release, causing CO₂ buildup and warming of the atmosphere. The cycle repeats, driving runaway warming until all water evaporates.

Important Implications (Within and Outside Our Solar System)

Looking within the solar system, the results offer an explanation for how a potentially habitable Venus could have transitioned into its current inhospitable state. If Venus had formed with an initial water inventory below this critical threshold of 20–50% of Earth’s ocean mass, then its carbon cycle would have become unbalanced, creating the CO₂ inferno seen on Venus today.

In future exoplanet studies, telescopes like the Habitable Worlds Observatory might detect ocean glint (specular reflection from liquid water) or measure land fraction from light curves. Finding a planet in the habitable zone but with limited ocean coverage would likely be bad news, as the planet could be quietly losing its grip on habitability. Astronomers may want to be more careful when looking at seemingly promising planets within their habitable zones.

Original astrobite edited by Natalie Price.

About the author, Jared Bull:

I am a 2nd-year PhD student at Johns Hopkins University. I study brown-dwarf variability and am interested in using time-series observations to uncover dynamic processes within their atmospheres. In my free time I like to read, cook, and do astrophotography.

The post Can Arid Planets Keep Their Cool? appeared first on AAS Nova.

An exoplanet is exerting a magnetic influence on its host star in a manner opposite to what astronomers normally observe, according to researchers at the Institute of Astrophysics of Andalusia (IAA-CSIC).

In a recent paper published in Science, IAA-CSIC researchers revealed how the exoplanet GJ 436 b is affecting its host star’s behavior in a never-before-seen way. In their work, the team developed a novel method to estimate an exoplanet‘s magnetic field strength, an important factor in determining habitability.

“Until recently it was thought that it was mainly the star that influenced the planet,” said Rafael Luque, an IAA-CSIC researcher and one of the new study’s co-authors.

Luque adds that his team’s results now offer “the clearest evidence to date of something that had already been suspected: that the opposite can also happen and that a nearby planet can alter the environment of its star.”

Life and Magnetic Fields

While the most obvious ingredients for a habitable planet are water and a breathable atmosphere, other elements are less apparent to us based on our human experience. Our planet’s invisible magnetic field protects us from the solar wind and holds the atmosphere together.

While Earth’s magnetic field guided the evolution of its atmosphere, Mars provides an example of what the loss of a magnetic field can do to an otherwise potentially habitable planet. When the Martian magnetic field disappeared four billion years ago as the planet’s core rapidly cooled, the atmosphere dissipated, followed by the loss of surface water.

As shown by the fates of two cosmic neighbors like Earth and Mars, a stable magnetic field is a key element of habitability.

Observing Exoplanet Magnetic Fields

Detecting magnetic fields on distant exoplanets has proven more challenging for astronomers than other observations, such as atmospheric composition. In their new paper, the IAA-CSIC team provides the first conclusive evidence of an exoplanet’s magnetic field, observed through its impact on the host star.

“In particular, we have observed that GJ 436 b, an exoplanet similar to Neptune that orbits very close to its star, causes regular changes in the brightness and energy emitted by the star at certain wavelengths,” explained lead author Daniel Revilla, an IAA-CSIC researcher who led the study as part of his doctoral thesis.

GJ 436 b provided the team with measurable signals that allowed them to infer that the exoplanet had a magnetic field and estimate its strength. The IAA-CSIC team analyzed sixteen years of spectroscopic observations of the GJ-436 system for their research, collected by the CARMENES instrument at the Calar Alto Observatory.

“Despite its smaller size, GJ 436 b would have a magnetic field between 2.33 and 27 times more intense than that of Jupiter,” said co-author Pedro J. Beloved, a researcher at the IAA-CSIC.

A Strange Interaction

In the paper, the researchers demonstrate how, in a very close orbit, a planet can flip the normal planet-star relationship, with the planet controlling the influence of gravity, radiation, and magnetic fields between the two. As the exoplanet interacts with its host star, it injects energy into the star’s chromosphere to produce a phenomenon like a stellar aurora.

This strange interaction has only been measured periodically, occurring in eight-year cycles, with observations in 2008, 2016, and 2024. Intriguingly, these years correlate to phases of the host star’s magnetic cycle, which the researchers believe affects the intensity.

“Until now, measuring the magnetic field of an exoplanet was extremely difficult,” Devilla concluded. “This property is key to knowing if a planet can protect its atmosphere and, ultimately, if it could harbor life.”

The paper, “Constraining an Exoplanet’s Magnetic Field Using Star-Planet Interactions,” appeared in Science on June 25, 2026.

Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.