The researchers propose that if a terrestrial planet has substantially less carbon dioxide in its atmosphere compared to other planets in the same system, it could be a sign of liquid water — and possibly life — on that planet’s surface.

What’s more, this new signature is within the sights of NASA’s James Webb Space Telescope (JWST). While scientists have proposed other signs of habitability, those features are challenging if not impossible to measure with current technologies. The team says this new signature, of relatively depleted carbon dioxide, is the only sign of habitability that is detectable now.

“The Holy Grail in exoplanet science is to look for habitable worlds, and the presence of life, but all the features that have been talked about so far have been beyond the reach of the newest observatories,” says Julien de Wit, assistant professor of planetary sciences at MIT. “Now we have a way to find out if there’s liquid water on another planet. And it’s something we can get to in the next few years.”

The team’s findings will appear in Nature Astronomy. De Wit co-led the study with Amaury Triaud of the University of Birmingham in the UK. Their MIT co-authors include Benjamin Rackham, Prajwal Niraula, Ana Glidden Oliver Jagoutz, Matej Peč, Janusz Petkowski, and Sara Seager, along with Frieder Klein at the Woods Hole Oceanographic Institution (WHOI), Martin Turbet of Ècole Polytechnique in France, and Franck Selsis of the Laboratoire d’astrophysique de Bordeaux.

Beyond a glimmer

Astronomers have so far detected more than 5,200 worlds beyond our solar system. With current telescopes, astronomers can directly measure a planet’s distance to its star and the time it takes it to complete an orbit. Those measurements can help scientists infer whether a planet is within a habitable zone. But there’s been no way to directly confirm whether a planet is indeed habitable, meaning that liquid water exists on its surface.

Across our own solar system, scientists can detect the presence of liquid oceans by observing “glints” — flashes of sunlight that reflect off liquid surfaces. These glints, or specular reflections, have been observed, for instance, on Saturn’s largest moon, Titan, which helped to confirm the moon’s large lakes.

Detecting a similar glimmer in far-off planets, however, is out of reach with current technologies. But de Wit and his colleagues realized there’s another habitable feature close to home that could be detectable in distant worlds.

“An idea came to us, by looking at what’s going on with the terrestrial planets in our own system,” Triaud says.

Venus, Earth, and Mars share similarities, in that all three are rocky and inhabit a relatively temperate region with respect to the sun. Earth is the only planet among the trio that currently hosts liquid water. And the team noted another obvious distinction: Earth has significantly less carbon dioxide in its atmosphere.

“We assume that these planets were created in a similar fashion, and if we see one planet with much less carbon now, it must have gone somewhere,” Triaud says. “The only process that could remove that much carbon from an atmosphere is a strong water cycle involving oceans of liquid water.”

Indeed, the Earth’s oceans have played a major and sustained role in absorbing carbon dioxide. Over hundreds of millions of years, the oceans have taken up a huge amount of carbon dioxide, nearly equal to the amount that persists in Venus’ atmosphere today. This planetary-scale effect has left Earth’s atmosphere significantly depleted of carbon dioxide compared to its planetary neighbors.

“On Earth, much of the atmospheric carbon dioxide has been sequestered in seawater and solid rock over geological timescales, which has helped to regulate climate and habitability for billions of years,” says study co-author Frieder Klein.

The team reasoned that if a similar depletion of carbon dioxide were detected in a far-off planet, relative to its neighbors, this would be a reliable signal of liquid oceans and life on its surface.

“After reviewing extensively the literature of many fields from biology, to chemistry, and even carbon sequestration in the context of climate change, we believe that indeed if we detect carbon depletion, it has a good chance of being a strong sign of liquid water and/or life,” de Wit says.

A roadmap to life

In their study, the team lays out a strategy for detecting habitable planets by searching for a signature of depleted carbon dioxide. Such a search would work best for “peas-in-a-pod” systems, in which multiple terrestrial planets, all about the same size, orbit relatively close to each other, similar to our own solar system. The first step the team proposes is to confirm that the planets have atmospheres, by simply looking for the presence of carbon dioxide, which is expected to dominate most planetary atmospheres.

“Carbon dioxide is a very strong absorber in the infrared, and can be easily detected in the atmospheres of exoplanets,” de Wit explains. “A signal of carbon dioxide can then reveal the presence of exoplanet atmospheres.”

Once astronomers determine that multiple planets in a system host atmospheres, they can move on to measure their carbon dioxide content, to see whether one planet has significantly less than the others. If so, the planet is likely habitable, meaning that it hosts significant bodies of liquid water on its surface.

But habitable conditions doesn’t necessarily mean that a planet is inhabited. To see whether life might actually exist, the team proposes that astronomers look for another feature in a planet’s atmosphere: ozone.

On Earth, the researchers note that plants and some microbes contribute to drawing carbon dioxide, although not nearly as much as the oceans. Nevertheless, as part of this process, the lifeforms emit oxygen, which reacts with the sun’s photons to transform into ozone — a molecule that is far easier to detect than oxygen itself.

The researchers say that if a planet’s atmosphere shows signs of both ozone and depleted carbon dioxide, it likely is a habitable, and inhabited world.

“If we see ozone, chances are pretty high that it’s connected to carbon dioxide being consumed by life,” Triaud says. “And if it’s life, it’s glorious life. It would not be just a few bacteria. It would be a planetary-scale biomass that’s able to process a huge amount of carbon, and interact with it.”

The team estimates that NASA’s James Webb Space Telescope would be able to measure carbon dioxide, and possibly ozone, in nearby, multiplanet systems such as TRAPPIST-1 — a seven-planet system that orbits a bright star, just 40 light years from Earth.

“TRAPPIST-1 is one of only a handful of systems where we could do terrestrial atmospheric studies with JWST,” de Wit says. “Now we have a roadmap for finding habitable planets. If we all work together, paradigm-shifting discoveries could be done within the next few years.”

“From 2009 to 2022, we conducted a series of field surveys in South China and collected a number of specimens of the Calotes versicolor species complex, and found that the population of what we thought was Calotes versicolor in South China and Northern Vietnam was a new undescribed species and two subspecies,” says Yong Huang, whose team described the new species.

Wang’s garden lizard (Calotes wangi) is less than 9 cm long, and one of its distinguishing features is its orange tongue.

“Calotes wangi is found in subtropical evergreen broad-leaved forests and tropical monsoon forests in southern China and northern Vietnam, mostly in mountainous areas, hills and plains on forest edges, arable land, shrub lands, and even urban green belts. It is active at the edge of the forest, and when it is in danger, it rushes into bushes or climbs tree trunks to hide. Investigations found that the lizards lie on sloping shrub branches at night, sleeping close to the branches,” says Yong Huang.

It is active from April to October every year, while in the tropics it is active from March to November or even longer, and eats a variety of insects, spiders, and other arthropods.

For now, the researchers estimate that the new species is not threatened, but they do note that in some areas its habitat is fragmented.

“In addition, their bodies are used medicinally and the lizards are also eaten,” they write in their research paper.

This is why they suggest that the local government strengthen the protection of their ecological environment and pay close attention to the population dynamics.

Simon Haeder, Ph.D., associate professor, analyzed data from an August 2023 survey of more than 2,000 dog and more than 1,400 cat owners to measure pet vaccination rates, perceptions of vaccines and support for pet vaccination requirements.

“Decreasing pet vaccination rates pose challenges to society for a number of reasons, including increased incidents of pet disease and death, increases in exposures for humans, the potential for further genetic adaptations of pathogens, as well as detrimental effects on veterinarians,” Haeder said. “Many individuals consider their pets as part of the family and increases in vaccine-preventable diseases may also affect the financial and emotional health of owners.”

The survey first asked respondents whether they owned a dog, a cat or both. Dog and cat owners were then surveyed about their pets’ vaccine status for five diseases each for dogs and cats. These included rabies for dogs and cats, canine parvovirus and canine distemper for dogs, and feline panleukopenia and feline Bordetella for cats. The respondents then responded with levels of support for vaccination requirements for each of the listed diseases. The survey also queried respondents about perceived safety, efficacy and importance of the various vaccines.

In addition to pet vaccine-specific questions, the survey asked respondents about their level of trust in scientists, support for human vaccination mandates for children, political ideology, religiosity, non-veterinary expenses and frequency of exposure of dogs to other dogs outside the household. Lastly, the survey measured perceptions of safety, efficacy and importance of human vaccines.

The survey found that an overwhelming majority of pet owners had vaccinated their dogs and cats against rabies, though cats were vaccinated less often than dogs. Other core vaccines had slightly lower, but still high uptake, while there appeared to be more hesitancy toward non-core vaccines. Core vaccines are generally recommended for all pets regardless of lifestyle.

Further analysis found that perceptions of importance, efficacy and safety of vaccines served as a reasonable predictor for vaccine hesitancy. Additionally, these perceptions show an association with attitudes toward vaccination requirements. Haeder’s analysis also found that pet owners without non-veterinary expenses such as boarding or training fees showed higher levels of vaccine hesitancy. Lastly, pet vaccination behaviors and perceptions appear to be less associated with political ideology than with human vaccines.

The findings of this study show a high level of confidence in vaccine safety, efficacy and importance for humans and pets. Additionally, the analysis found relations between vaccine hesitancy in humans and animals, with support for animal vaccine requirements being strongly associated with similar requirements for humans. This indicates the potential for spillover effects and the importance of further focus on vaccine hesitancy in humans and animals in research and public health efforts in the future.

“Concerns about growing hesitancy remain and should be taken seriously, for both pets and humans, before the United States falls below important thresholds to prevent major outbreaks of vaccine-preventable diseases,” Haeder said.

“The more reindeer ruminate, the less additional non-REM sleep they need,” says first author and neuroscientist Melanie Furrer of the University of Zurich. “We think it’s very important that they are able to save time and cover their sleep and digestive needs at the same time, especially during the summer months.”

Light-dark cycles are absent in the Arctic during winter and summer, and previous studies showed that Arctic-dwelling reindeer don’t display circadian behavioral rhythms during these seasons, though they tend to be more active during the daytime during the spring and autumn equinox, when light-dark cycles are present. However, it was unknown whether these seasonal differences also impacted how much — and how well — reindeer sleep.

To investigate the influence of seasonal light-dark cycles on reindeer sleep patterns, the researchers performed non-invasive electroencephalography (EEG) on Eurasian tundra reindeer (Rangifer tarandus tarandus) in Tromsø, Norway (69°N), during the autumn equinox, summer solstice, and winter solstice. The reindeer, who were all adult females, were part of a captive herd at UiT The Arctic University of Norway in Tromsø, and the experiments were conducted in indoor stables with controlled lighting, unlimited food, and constant temperature.

They found that reindeer slept approximately the same amount during winter, summer, and autumn, despite the fact that they were much more active during the summer. This is in contrast to other species who change the amount they sleep in response to environmental conditions. On average, the reindeer spent 5.4 hours in non-REM sleep, 0.9 hours in REM sleep, and 2.9 hours ruminating during a given 24-hour period, regardless of season.

“The fact that reindeer sleep the same amount during winter and summer implies that they must have other strategies to cope with limited sleep time during the arctic summer,” says Furrer.

One possible strategy is the opportunity for rest during rumination — the re-chewing of partially digested food, which is an important component of digestion for reindeer and other ruminants. Domestic sheep, goats, cattle, and Lesser mouse-deer have all been previously observed to produce sleep-like brain waves during rumination, but it was unclear whether rumination could serve a similar restorative function to sleep.

The researchers found that the reindeer’s EEG readings during rumination resembled brainwave patterns that are indicative of non-REM sleep including increased slow-wave activity and sleep spindles. Sleeping and ruminating reindeer also displayed similar behavior, tending to quietly sit or stand during both activities, and were less reactive to disturbances such as a neighboring reindeer sitting down or getting up — reindeer directly responded to these disturbances (by looking toward the neighboring reindeer) 45% of the time if they were awake, but only 25% of the time if they were ruminating, and 5% of the time if they were in non-REM sleep.

Next, the researchers tested whether rumination could reduce the reindeer’s drive to sleep by depriving the reindeer of sleep for 2 hours and measuring their brain waves during sleep before and after this deprivation. Following sleep deprivation, the reindeer’s EEG readings showed increased slow-wave activity, which is indicative of a build-up of “sleep pressure” — the unconscious biological drive for more and deeper sleep — suggesting that reindeer experience deeper sleep following sleep deprivation.

However, when the reindeer ruminated, this slow-wave activity was decreased during subsequent sleep, and the more they ruminated, the more the slow-wave activity decreased. “This suggests that rumination reduces sleep pressure, which could benefit the reindeer because it means they don’t have to compromise on sleep recovery when they spend more time ruminating,” says Furrer.

This is especially important during the summer, because the more they eat, the more time the reindeer need to spend ruminating. “Rumination increases nutrient absorption, so it’s crucial for reindeer to spend enough time ruminating during the summer in order to gain weight in anticipation of winter,” says Furrer.

Since reindeer appear to sleep while ruminating only some of the time, follow-up studies should compare the impact of rumination while sleeping with rumination while awake and would also ideally measure reindeer behavior and sleep in more natural outdoor conditions, the researchers say. However, such measurements would require surgically implanted EEG sensors rather than the non-invasive surface electrodes used in this study.

“Another thing we could add is to look at young reindeer,” says Furrer. “We know sleep need is much higher in young children and babies compared to adults, so it would be interesting to look at sleep in younger reindeer.”

This research was supported by the Swiss National Science Foundation and UiT The Arctic University of Norway.