Mars holds a special place in the Solar System. It represents marginal habitability. This means it transitioned from warm and wet and potentially hospitable, to cold and dry and inhospitable.

What can its transition tell us about exoplanet habitability?

New research to be published in the Planetary Science Journal examines the question. It’s titled “Mars as an Exoplanet: Lessons from a Planet at the Edge of Habitability.” The lead author is Stephen Kane, Professor of Planetary Astrophysics in the Earth & Planetary Sciences Dept. at the University of California, Riverside. The research is currently available at arxiv.org.

“Mars is the Solar System’s canonical small, rocky planet that transitioned from early geologic activity and surface liquid water to a cold and arid planet with a thin, cold, CO-dominated atmosphere,” the authors write. “The evolution of Mars, in the context of such planetary parameters as size, mass, atmosphere, insolation flux, magnetosphere, and impact history, harbor important diagnostics regarding the development and sustainability of habitable surface conditions.”

*This figure shows the planetary mass and radius data for confirmed exoplanets that have measurements extracted for both properties, extracted from the NASA Exoplanet Archive on 2025, December 31. The data are color-coded in proportion to the flux received from their host stars. The Solar System terrestrial planets are shown as stars. The shaded region indicates the sub-Earth regime. Image Credit: Kane et al. 2026. PSJ*

Our understanding of the exoplanet population has grown enormously in recent years. In exoplanet surveys, small rocky worlds are common and outnumber larger gas planets. But while we know they exist in large numbers, we lack a detailed understanding of their climates, their volatile budgets, and their long-term potential for habitability. According to the authors, Mars can help us understand its exoplanet cousins.

They point out that though size is a basic property of rocky planets, and a good starting point for understanding them, it doesn’t dictate how a planet evolves. “Venus, Earth, Mars, and even the Moon each underwent distinct volatile, tectonic, and atmospheric trajectories despite sharing the same stellar environment, illustrating that planet size alone does not uniquely determine planetary evolution,” they explain.

In this research, the authors synthesize research into how different aspects of Mars—including volatile delivery and loss, photochemistry, climate evolution, magnetism, and other factors—can help our overall understanding of exoplanets and their processes.

“Exoplanet studies often use Earth properties as standard units of measurements, particularly for those relevant to describing the capabilities of exoplanet detection
methods,” the authors write. Mars has many similar properties to Earth, but its diffferences are what’s important in this work.

*These schematic cross sections of Earth and Mars show the major internal components and atmospheric components to scale. For simplicity, oceanic and continental crust for Earth are not distinguished, nor is the interior structure of Earth’s mantle shown. Image Credit: Kane et al. 2026. PSJ*

First of all, Mars formed differently from Earth. It’s formation was rapid at first, then stalled at a sub-Earth mass. The authors describe it as a “stranded planetary embryo” instead of the result of later giant impacts.

The planet’s mass is important in its evolution, which isn’t surprising. “Mars occupies an important position in comparative planetology, since it is both a geologically rich world with a documented history of surface habitability, and a representative example of how small rocky planets can evolve toward atmospheric loss and climatic decline,” they write.

Mars can serve as a framework for understanding rocky exoplanets. One of the main conclusions is that Mars shows how planetary habitability isn’t a static condition. The authors describe it as “a time-dependent outcome governed by competing processes.”

For example, early Mars was volcanic, and released volatiles built up a thick atmosphere that trapped heat. But as its interior cooled and its dynamo stopped, atmospheric escape led to cooling and eventual loss of habitability. “These coupled processes can define a pathway that may be common for Mars-mass planets,” the authors write.

According to our understanding of Mars, habitability is likely to be fleeting more often than not, and Earth shines as a rare example of long-term habitability. “In
this context, Mars represents the edge of the habitable regime, being large enough to host transiently clement conditions, but small enough that atmospheric retention
and replenishment and long-term climate regulation are not guaranteed,” the authors write.

While Mars-mass planets are widely detected, there are shortcoming in those observations. “Our discussion of exoplanet demographics have shown that, while terrestrial-size planets are abundant, confirmed Mars-mass planets with well-constrained masses and radii remain relatively rare, largely due to detection shortcomings,” the authors write. That will change when the Nancy Grace Roman Telescope and its microlensing survey goes live.

As we discover more Mars-mass planets with well-measured constraints, we’re also developing future telescopes that get better at observing exoplanets. “Direct imaging and thermal emission studies, particularly with next-generation facilities, will ultimately determine whether such planets commonly retain thin CO2 atmospheres, undergo desiccation, or exhibit transient volatile cycles,” the researchers explain.

The key idea is that scientists can use what they learn about Mars to understand these observations. “Mars missions will continue to measure atmospheric escape rates, volatile inventories, and climate feedbacks with a level of detail unattainable for exoplanets, while exoplanet surveys contextualize Mars within a broader statistical population,” the authors write.

The researchers explain that as Mars exploration and exoplanet characterization converge, it will deliver an effective new way to better understand the large numbers of small rocky worlds. Scientists will better understand key properties of exoplanets, like the mass necessary to sustain geological activity like plate tectonics. They’ll also develop a better understanding the stellar environment and how it shapes atmospheric survival, as well as other planetary characteristics that shape habitability.

“Within this framework, Mars provides a fundamental benchmark for evaluating the diversity, evolution, and potential habitability of rocky planets throughout the Galaxy,” the authors conclude.

A coalition of conservation groups wants Southern California to get 85% of its water locally, up from the 50% it gets now, by 2045, and says a new plan shows how.

It’s urging state leaders to scrap plans for a 45-mile tunnel beneath the Sacramento-San Joaquin River Delta and consider asking voters to approve a bond measure to fund local water solutions. The 34-page strategy was released as critical decisions loom for local officials, California’s next governor and legislators.

Over the last century, Southern California has grown and thrived thanks to giant aqueducts it built to bring water from hundreds of miles away — the Eastern Sierra, the Colorado River and Northern California.

But with water costs rising and climate change jeopardizing these distant sources, there is growing interest in finding ways to get more water locally.

The allied groups are calling for recycling more wastewater, capturing more stormwater, improving efficiency and cleaning up contaminated groundwater.

“We have to prioritize our investments, and prioritizing them in local water makes the most sense,” said Bruce Reznik, executive director of the group Los Angeles Waterkeeper.

The coalition includes fishing groups, environmental organizations and Northern California’s Winnemem Wintu Tribe.

Its plan calls for a “new urban water renaissance” in California that prioritizes local water. This approach would reliably yield more and cost far less than Gov. Gavin Newsom’s proposed Delta Conveyance Project beneath the Delta.

The state estimated in 2024 the tunnel would cost $20.1 billion, but opponents say it could cost three to five times more.

“Local water is reliable, it’s more affordable, and it’s more flexible, so that we’re not committing California ratepayers to higher bills that they don’t need,” said Kyle Jones, a water expert and consultant who helped prepare the plan for the coalition.

Southern California imports about half of its water from other regions.

The coalition’s plan says the region can secure up to 2 million acre-feet of local water per year. It estimates the costs of more conservation and efficiency, more stormwater and groundwater cleaning, and more water recycling at $44 billion over two decades. The Delta tunnel, in contrast, could cost $60 billion to $100 billion, it says.

Whether the tunnel project is ultimately built may hinge on whether large water agencies, including the Metropolitan Water District of Southern California, decide to participate and pay for it.

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1. Cranes rise above the Donald C. Tillman Water Reclamation Plant in Van Nuys. 2. When completed, Los Angeles will nearly double recycled water for 500,000 residents. 3. Storage tanks sit behind a fence before being placed in the ground at the plant. (Myung J. Chun / Los Angeles Times)

“Metropolitan Water District really does have a significant choice on it, that not just impacts their ratepayers but impacts every single person in the state,” said Barbara Barrigan-Parrilla, executive director of the group Restore the Delta. “Are we going to spend $20, $60, maybe upward to $100 million on a tunnel? Or are we going to invest significant money in local solutions that provide water resiliency and sustainability for everyone in California? That is what is at stake right now.”

The Metropolitan Water District already is planning a large new facility in Carson to transform wastewater into purified drinking water. Los Angeles and San Diego are also building water recycling plants.

“At the same time, water imported from the northern Sierra and the Colorado River provides the foundation of water supply reliability for Southern California,” said Shivaji Deshmukh, the MWD’s general manager.

He noted that the MWD invests in water efficiency and capturing stormwater, and has helped reduce per-person water use by more than 40% since 1990.

The agency’s 38-member board last year adopted a climate adaptation strategy that sets goals for lining up additional water.

Los Angeles city leaders and L.A. County supervisors have also set goals for becoming more locally self-sufficient.

The advocates who wrote the policy plan said these efforts should accelerate and expand. They pointed out that the Colorado River’s reservoirs are falling to perilously low levels, and native fish in the Delta are in decline as the pumping of water takes an ecological toll.

“Climate change is exacerbating the challenges in those ecosystems, meaning that less and less water will be available to import,” said Ashley Overhouse, water policy advisor for the group Defenders of Wildlife. “All the while, the cost of water is continuing to rise.”

About 20 other environmental groups endorsed the coalition’s strategy.

“We have got to do a better job in the next 100 years than we did in the last 100 years, if we truly want to create a place of abundance once again,” said Frankie Myers, a member of the Yurok Tribe in Northern California. “This idea that we can steal … and divert water however we want with no consequences has got to end.”

Construction continues at the Donald C. Tillman Water Reclamation Plant in Van Nuys in October 2025.

(Eric Thayer / Los Angeles Times)

Benjamin Bass, a UCLA scientist who studies how climate change is affecting the Colorado River and other water sources, joined the group as they presented their proposal in an online briefing.

“Traditional sources for imported water are less reliable than they used to be,” Bass said. “The most reliable source of water in the future is local water.”

Other experts have reached similar conclusions.

Researchers at the Pacific Institute, a water think tank in Oakland, have examined improvements such as fixing leaks in pipes, switching out inefficient washing machines and toilets, and replacing thirsty lawns with plants suited to the state’s Mediterranean climate.

In a 2022 report, they found that a set of standard practices and technologies could reduce total urban water use by 30% or more.