Red dwarfs make up the vast majority of stars in the galaxy. Such ubiquity means they host the majority of rocky exoplanets we’ve found so far – which in turn makes them interesting for astrobiological surveys. However, there’s a catch – astrobiologists aren’t sure the light from these stars can actually support oxygen-producing life. A new paper, available in pre-print on arXiv, by Giovanni Covone and Amedeo Balbi, suggests that they might not – when it comes to stellar light, quality is just as important as quantity. And according to their calculations, Earth-like biospheres are incredibly difficult to sustain around red dwarfs.

Their argument is based on the concept of exergy – a measurement of the maximum amount of useful work that can be extracted from a radiation field. In other words, it measures the thermodynamic quality of the light, not just the raw energy contained in it. Typically, when measuring the “habitable zone” of stars, astrobiologists look at the total number of photons, specifically in the visible light range between 400 and 700 nanometers of wavelength.

So what “useful work” does light do on exoplanets? Perhaps the most important is breaking apart water. This process, known as “water oxidation” is a kinetic bottleneck in the process of photosynthesis, and creates the oxygen expected to be seen in biosignatures. However, to do this, biological systems require a significant amount of kinetic energy to perform this chemical reaction. And red dwarfs have two strikes against them when it comes to providing that energy.

Fraser talks about habitable planets around Red Dwarfs

Red dwarfs are cool, and their light is heavily red-shifted into the infrared. Not enough of their photons pack enough energy to reach the threshold needed to split water. But even the ones that do have a smaller percentage of their energy that can actually be converted into useful chemical work. This one-two combination puts a huge dent in the potential of oxygenic life forming around red dwarfs. By comparison, the exergy available to drive water oxidation around Sun-like stars is around five times higher.

Astrobiologists are an optimistic bunch, though, so their immediate response to this concern would be – maybe life evolved around those stars to adapt to these higher infrared environments. Could they use longer, lower-energy infrared wavelengths under the skies of a red dwarf? The short answer is no, due to something called the red limit. This is the longest wavelength of light capable of supporting photosynthesis. The authors argue that this isn’t a set value, but an emergent property determined by a star’s spectrum, the planet’s atmosphere, and a targeted chemical reaction – in this case the water oxidation.

They estimate that for red dwarfs the red limit is 0.95 um, whereas for Sun-like stars its closer to 1.0 um. In practice, that means life cannot simply shift their primary absorption bands deeper into the near-infrared to adapt to their less powerful star. Another concern has to do with the evolution of life on one of these planets. Anoxygenic bacteria can effectively harvest infrared light. If allowed to proliferate, they could out-compete oxygenic bacteria, and the world would never experience a “Great Oxidation Event” equivalent to what happened on Earth. Without copious amounts of oxygen in the atmosphere, multicellular life would be severely hindered, if not outright eliminated altogether.

Fraser has a few videos on this topic, showing that there’s been an ongoing debate.

Taking all of this into account paints a bleak picture for the possibility of life around red dwarfs. But let’s not rule it out entirely. Currently, the Earth’s biosphere only uses about three orders of magnitude below the maximum thermodynamic – proof that life itself is wildly inefficient. But even so, the conditions surrounding red dwarfs that would be favorable for life are likely extremely rare. This paper proves that our time searching for an oxygen-rich alien forest might be better spent around stars like our Sun, rather than chasing the statistical rarity of a flourishing biosphere surrounding a red dwarf.

Learn More:

G. Covone & A. Blabi – Photosynthetic exergy I. Thermodynamic limits for habitable-zone planets

UT – Red Dwarfs Are Too Dim To Generate Complex Life

UT – Habitable Zone Planets Around Red Dwarfs Aren’t Likely To Host Exomoons

UT – New Research Suggests Red Dwarf Systems are Unlikely to Have Advanced Civilizations

With desert cities like Phoenix and Tucson bracing for their allotments of Colorado River water to be slashed dramatically, San Diego County’s water agency could for the first time sell some of its water to other states by drawing on its ample supplies from the nation’s largest desalination plant.

The San Diego County Water Authority’s board unanimously approved an initial agreement last week to consider selling some of its water to Arizona and Nevada, where cities that depend on the over-tapped Colorado River are expected to face substantial cuts in water supplies.

General Manager Dan Denham said the agreement, if approved by other agencies, could clear the way for the first-ever interstate transfers of Colorado River water starting next year.

“It’s just a different way of managing water in the West,” Denham said. “I think it has to happen now, and it has to happen because of the situation on the river.”

The Colorado River provides water for farms, cities and tribal communities across seven states and northern Mexico. Its reservoirs have declined dramatically over the last 25 years as drought compounded by climate change has shrunk the river flow.

Negotiators for the seven states, despite extensive talks, remain at odds on the water cutbacks each should accept to prevent reservoirs from declining further.

Officials at San Diego County Water Authority plan to consider selling water across state lines by letting agencies in Arizona and Nevada purchase some of its Colorado River supplies. The revenue generated, the San Diego County agency says, would be used to reduce costs for its ratepayers.

The agreement — technically a memorandum of understanding — first would need to be approved by the Metropolitan Water District of Southern California, the federal government and agencies in Arizona and Nevada. Then, various water agencies would need to negotiate the details.

The San Diego County Water Authority serves as a wholesaler, delivering water to 22 cities and other agencies that serve 3.3 million people.

The authority is in a position to sell a portion of the region’s water because it has invested heavily in securing additional water supplies from the Claude “Bud” Lewis Carlsbad Desalination Plant, which since 2015 has been drawing in seawater and churning out a portion of the area’s drinking water. The authority secured additional water through a 2003 agriculture-to-urban transfer deal. These and other investments have brought San Diego County plentiful water — though as a result, the region also has some of the most expensive water in the state.

The Water Authority purchases water from the Carlsbad desalination plant under a 30-year agreement. But the plant is currently operating at less than full capacity, Denham said, and its output could be increased to provide a larger share of the region’s water if agencies in other states bought some of the Water Authority’s Colorado River water. Essentially, it’s too expensive to run the plant at full capacity at this time given the availability of other more economical supplies, but out-of-state money could make it worth the agency’s while.

Conservation efforts have also reduced the water needs of many of the cities that the Water Authority serves. And in the next few years, the area will also start recycling wastewater at new facilities, including San Diego’s Pure Water project as well as other recycling projects in Oceanside and eastern San Diego County.

If other states and agencies sign on, Denham said the Water Authority is prepared to sell up to 10,000 acre-feet of water starting next year. That’s nearly 5% of the Las Vegas area’s current water use.

In future years, he said, that could increase to 25,000 acre-feet or more. And with additional investment in upgrades, the Carlsbad desalination plant could be expanded to transform more seawater into drinking water, thereby freeing up additional water to be traded to cities that need it.

“It’s in everyone’s best interest to make this work,” Denham said.

Gov. Gavin Newsom has supported the idea, telling governors of the other six states in a recent letter that California would welcome joint investments in water recycling and desalination.

Denham said Scott Cameron, the Trump administration’s acting head of the U.S. Bureau of Reclamation, has also supported the idea.

The board of the Metropolitan Water District has yet to vote on the proposed agreement.

“I believe these kinds of concepts are what we need to do in regards to thinking about water resources, not from political boundaries, but as a region,” said Shivaji Deshmukh, the district’s general manager.

If the San Diego County Water Authority eventually reaches deals with other agencies, the amount of water sold will be small compared to the gaping water shortfall that is pushing reservoir levels lower along the Colorado River. But Denham said if it works, this approach could be a first step in showing that deals between states can help ease the water shortages.