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There might be a type of exoplanet without dry land. They’re called “Hycean” worlds, a portmanteau of ‘hydrogen’ and ‘ocean.’ They’re mostly or entirely covered in oceans and have thick hydrogen atmospheres.

They’re intriguing because their atmospheres keep them warm enough to have liquid water outside of the traditional habitable zones. If they do exist, scientists think they’re good candidates to support microbial life.

Hycean worlds are hypothetical, but there is some evidence that they exist. The Kepler mission detected many candidates and provided foundational evidence for their existence. However, it didn’t detect any with certainty.

More recently, JWST observations also supported the idea. The space telescope detected carbon dioxide and methane in the atmosphere of a candidate Hycean world called K2-18b. Both of those molecules can be biosignatures of microbial life under similar conditions as Earth’s oceans.

This infographic shows the chemicals the JWST detected in the atmosphere of K2-18b. In addition to the carbon-bearing molecules methane and carbon dioxide, it detected the potential biosignature dimethyl sulphide. Image Credit: JWST/STScI

New research published in the Monthly Notices of the Royal Astronomical Society examines the potential Hycean worlds hold for the evolution of life and how life might depend on these worlds’ thermodynamic conditions. It’s titled “Prospects for Biological Evolution on Hycean Worlds.” The authors are Emily G Mitchell and Nikku Madhusudhan, both from the University of Cambridge.

“The search for extraterrestrial life is one of the most fundamental quests in human history,” the authors write. “An important recent development in this direction is the possibility of Hycean worlds, which increase both the numbers of potentially habitable planets and the ability to detect biosignatures in their atmospheres.”

Research shows that Hycean Worlds can provide both the chemical and the thermodynamic conditions necessary for microbial life to persist in their oceans. In this research, the authors used the metabolic theory of ecology (MTE) to explore how simple life might evolve in Hycean Worlds under different temperature conditions. In simple terms, MTE says that an organism’s metabolic rate is fundamental to its ability to persist and thrive. It applies to individual processes and community and population processes. A key idea behind MTE is that temperature strongly influences metabolic rates.

Previous studies show that when temperatures in a habitable environment increase, biological activity increases up to a point. In this research, Mitchell and Madhusudhan investigate how ocean surface temperatures affect Earth-like single-celled life and how long it takes them to originate on Hycean Worlds. They also explore how different temperatures affect the detectability of biosignatures.

“This work, in turn, has observable consequences for prominent biosignatures on such planets, considering that unicellular phytoplankton are a major source of key biomarkers in the Earth’s atmosphere, such as dimethyl sulphide, which may be observable in Hycean atmospheres,” the researchers write in their paper.

Dimethyl sulphide is strongly linked to phytoplankton and has a unique spectral signature that the JWST can detect in exoplanet atmospheres.

The researchers focused on several key phytoplankton groups that are abundant on Earth and produce biosignature gases in its atmosphere. Among them are Cyanobacteria (blue-green algae), Methanococccea (a methanogen), and diatoms, which generate as much as 50% of Earth’s oxygen each year. They paid special attention to Aquificota.

Aquificota is a phylum of bacteria named after an early genus in the group Aquifix. Its members are found in fresh water and oceans and can produce water by oxidizing hydrogen.

“In order to illustrate how evolutionary rates change with temperature over planetary timescales, we have calculated the evolutionary rates for an example organism (Aquifix) over the last 4.3 billion years,” the paper states. They used Aquifix because it’s a strong analogue for some of Earth’s first life.

The researchers showed that even marginal changes in Earth’s ocean surface temperature compared to the surface temperature over evolutionary timescales significantly change the origination time and evolutionary rates of important species of simple life. “For example, a 10 K increase relative to Earth
leads to evolutionary rates which are over twice as fast, while a decrease of 10 K halves them,” the authors explain.

They found that warmer oceans can accelerate the rate of evolution, allowing key unicellular groups like archaea and bacteria to appear as early as 1.3 billion years after the origin of life. This indicates that higher temperatures drive a faster progression to complex life. “This increased rate has a significant impact on the origination times of unicellular groups such that for an increase of 10K of surface temperature, all of the major groups will have originated by 1.19 Gyr post-Origin of Life (OlL) and all the key phytoplankton groups by 1.28 Gyr,” the authors write.

This figure from the research shows the effect of temperature on the origination times of major clades. The origination time on Earth of each group is marked with a forward arrow. Red indicates increased
temperature by +10 K, and blue indicates decreased temperature by -10 K. “We find that an increase in the surface temperature of 10K results in all the phytoplankton groups originating with 1.3 Gyr of the OoL,” the authors explain. Cyanobacteria appear particularly early, only 0.25 billion years after the Origin of Life. Image Credit: Mitchell and Madhusudhan 2025.

The reverse is also true. The researchers found that cooler temperatures delay the appearance of key lifeforms by up to several billion years. That could mean that complex life takes longer to appear. “In contrast, a decrease of 10K of median surface temperature severely limits the origination rates, such that by 4 Gyr post-OoL, only Bacteria and Archaea will have evolved but not oxygenic photosynthesis or Eukaryotes,” the authors write.

In that case, it would also affect the appearance of observable biosignatures, and their intensity and ease of detection.

One of their central findings is that only a marginal range of environmental conditions allows for a large range of evolutionary rates and origination times. “First, given the wide range of possible atmospheric conditions in Hycean worlds, an equally wide diversity in microbial life could be expected,” they write. “In particular, the origination of new clades in warm Hycean worlds can happen significantly faster than on Earth.”

If Hycean worlds exist, this research suggests that they could be “rippling with life,” as Carl Sagan put it, on shorter timescales than Earth.

An artist’s illustration of a Hycean World. Image Credit: By Pablo Carlos Budassi – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=135998139

The candidate Hycean worlds we know of are thought to have warmer oceans than Earth. So, by extension, the candidate Hycean World K2-18 b, which is only 2.4 billion years old, could have the conditions necessary for originating and sustaining key unicellular groups. That means that it, and others like it, are good targets in the search for biosignatures.

The authors offer a couple of caveats to their results. They considered only a fairly narrow range of temperature and physical conditions based on Earth. In reality, habitable extraterrestrial planets could exhibit a much wider range. “Future work in this direction could explore a range of other conditions, including the effect of gravity, pressure, larger temperature variations and other environmental factors,” the researchers write in their conclusion.

We don’t know if Hycean Worlds are real. Some scientists think that their hydrogen-rich atmospheres might be unstable. There are also concerns about radiation exposure inhibiting life and atmospheric chemistry working against biochemical processes. The formation pathways for these worlds are also unclear, as are the mechanisms for generating and sustaining their atmospheres.

However, if they do exist, this study makes one thing clear: For different surface temperatures, a warm planet could have a more complex biosphere at a relatively young age, and a cooler one could have a simpler biosphere at a later age.

In the end, we aren’t travelling to any of these worlds, so detecting biosignatures is the name of the game.

“Such biospheres with varied levels of complexity can impact the detectability of life on them, such that warmer planets have the potential to show strong atmospheric biosignatures,” the researchers conclude.

The post Could Ocean Worlds Support Life? appeared first on Universe Today.

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Kepler was one of the most successful exoplanet-hunting missions so far. It discovered 2,600 confirmed exoplanets – almost half of the total – in its almost ten years of operation. However, most data analysis focused only on one of the 150,000 targets it “intended” to look at. While it was making those observations, there were a myriad of background stars that also had their light captured incidentally. John Bienias and Robert Szabó of Hungary’s Konkoly Observatory have spent a lot of time looking at those background stars and recently published a paper suggesting there might be seven more exoplanet candidates hiding in the data.

As with many space telescope missions, Kepler’s dataset is open to the public. NASA maintains a database with the raw data collected during the space telescope’s observations, and researchers are free to download it and analyze it as they see fit.

Plenty of interesting things are hiding in that data that were overlooked by the more than 3136 peer-reviewed scientific papers that have utilized Kepler’s data. In the past, the authors have published other documents using the same datasets that described eclipsing binary stars and RR Lyrae stars, a type of pulsating variable star already existing in the data.

Fraser discusses the end of Kepler’s mission.

But while looking for more data on another paper about longer-period versions of those phenomena, they came across several stars whose light curve variability indicated something different – a planet passing in front of them. These “transits”, as they are called, are one of the most common ways to identify exoplanet candidates, and have been used for decades, but this might be the first time they’ve been used on some of the 500,000 background stars in Kepler’s data.

That might be because the data is patchier, as the telescope was not focused on the stars in the background, making this resolution more difficult. However, difficult does not mean impossible, and plenty of software solutions have been developed in the six years since the end of Kepler’s primary mission to help facilitate crunching large sets of data to look for planets around other stars.

One such system that has been around for a while is the Lomb-Scargle algorithm, developed in the 1970s and 80s and designed to detect periodic signals within time-series data. This algorithm is a valuable step in finding both the eclipsing binaries the authors were initially looking for and the exoplanet candidates they recently described.

Fraser discusses Kepler’s newest successor – MAUVE.

Other, more modern tools, proved more finicky, such as PSFmachine. This software package is designed to “deblend” light curves in Kepler’s data. Light curves are critical to exoplanet hunting as they show how the brightness of an object changes over time. However, in Kepler’s background, multiple stars might be overlapping, causing a blending of their light curves. PSFmachine is designed to deal with that problem. However, the authors described several issues in using the software, including its inability to create any stand-alone curves in one case. This seemed to be due to the placement of the stars compared to Kepler’s aperture (i.e., they were in the background) and the relatively small variations seen in the data.

Another tool developed near the end of Kepler’s mission was Pytransit, a Python-based software package that estimates the transmit models of light curves, including period, sizes, and orbital eccentricity. Candidate stars were also cross-referenced with the dataset from Gaia, which is designed to capture data about stars. 

Utilizing all the tools, the authors identified seven exoplanet candidates. All were hot Jupiters, with sizes between .89 and 1.52 Jupiter’s radius and orbits between .04 and .07 AU. They also checked to see if any of those dips in light curves might have been caused by second planets orbiting the same star but came up empty-handed.

While seven additional candidate exoplanets might not seem like a lot compared to the 2,600 confirmed ones Kepler already found, combing over already released data shows how much more helpful context is sometimes publicly available if a researcher knows where and how to look. As more powerful software packages and analytical tools are developed, there will undoubtedly be more discoveries coming out of older data sets like Kepler’s for some time to come.

Learn More:
J Bienias & R Szabó – Background exoplanet candidates in the original Kepler field
UT – Old Data from Kepler Turns Up A System with Seven Planets
UT – This is Kepler’s Final Image
UT – It’s Over For Kepler. The Most Successful Planet Hunter Ever Built is Finally out of Fuel and Has Just Been Shut Down.

Lead Image:
Artist’s impression of Kepler
Credit – NASA Amex / JPL-Caltech/T Pyle

The post Even More Planets Were Hiding in Kepler’s Fields appeared first on Universe Today.