K2-18b May Not Be Habitable After All

Cartoon showing a variety of exoplanet types. Figuring out whether a planet is rocky or gaseous can be a challenge, as is the case for K2-18b.  Credit: NASA/JPL-Caltech/Lizbeth B. De La Torre

Exoplanet K2-18b made headlines when researchers reported that JWST observations of the planet were consistent with a habitable ocean world. Now, another team has published a different interpretation of the data, suggesting that the purported water world is instead a gas-rich planet with no habitable surface.

An artist’s impression of K2-18b as an ocean world.
Credit: NASA, ESA, CSA, Joseph Olmsted (STScI)

Everybody Wants to Rule the Find a Habitable World

The small, cool star K2-18 is home to two planets, one of which has garnered plenty of attention in the decade since its discovery. Recently, JWST data of K2-18b, an 8.6-Earth-mass planet, revealed the presence of atmospheric carbon dioxide and methane. Some researchers have interpreted these data, coupled with the non-detection of ammonia, water, and carbon monoxide, to mean that K2-18b is a Hycean world: a rocky planet covered in oceans.

To make matters more interesting, the same research team reported weak evidence for dimethyl sulfide, a compound that on Earth forms almost exclusively due to life. This led many onlookers to the eyebrow-raising conclusion that K2-18b is not just habitable but inhabited.
These intriguing interpretations, however, are far from settled. Is K2-18b truly a habitable ocean world, or could alternative explanations fit the JWST data equally well?

Example simulation output for K2-18b as a gas-rich planet without a habitable surface.
Credit: Wogan et al. 2024

Water World or Gas Planet?

A team led by Nicholas Wogan (NASA Ames Research Center and University of Washington) tackled this question by applying two sets of models to the JWST data. The first set describes rocky planets with surface oceans, with and without life, and the second set describes gaseous planets without a surface and without life. The models predict the planet’s photochemistry — chemical reactions in the atmosphere driven by photons from the host star — and climate.

Wogan’s team found that K2-18b is unlikely to be a lifeless water world, since this type of planet wouldn’t contain enough methane in its atmosphere to produce the signal seen in the JWST observations. Intriguingly, a water world with microbial life is more promising: acetotrophic methanotrophs — a tongue-twisting name for simple methane-producing organisms — may be able to produce the supply of methane seen in the planet’s atmosphere.

JWST transmission spectra (black and gray points with error bars) and modeled spectra for K2-18b as a lifeless ocean world (top left), an ocean world with life (bottom left), and a lifeless gas-rich planet (bottom right). Click to enlarge. Credit: Wogan et al. 2024

Not So Fast…

As exciting as this sounds, Wogan and collaborators found that the uninhabitable gas-rich exoplanet model fits the JWST data equally well, and this model may pose fewer problems. Not only does the ocean-world model require life to explain its atmospheric composition, it’s also hard to reconcile the necessary cool surface temperature with the high likelihood of the planet experiencing a runaway greenhouse effect.

This isn’t the last word on K2-18b — there are features in the planet’s spectrum that aren’t well fit by a lively ocean world or a lifeless gas-rich planet, and both models have their challenges. Future data from JWST might dredge up a detection of ammonia, which would point to a gaseous planet, or dimethyl sulfide, which would tilt the scales considerably toward an inhabited water world. In the meantime, the hunt for habitable planets goes on.

By Kerry Hensley

Citation

“JWST Observations of K2-18b Can Be Explained by a Gas-Rich Mini-Neptune with No Habitable Surface,” Nicholas F. Wogan et al 2024 ApJL 963 L7. doi:10.3847/2041-8213/ad2616

Source: American Astronomical Society/AAS-Nova

---

New class of habitable exoplanets are 'a big step forward' in the search for life

Hycean Planets

A new class of exoplanet very different to our own, but which could support life, has been identified by astronomers, which could greatly accelerate the search for life outside our Solar System.

In the search for life elsewhere, astronomers have mostly looked for planets of a similar size, mass, temperature and atmospheric composition to Earth. However, astronomers from the University of Cambridge believe there are more promising possibilities out there.

The researchers have identified a new class of habitable planets, dubbed ‘Hycean’ planets – ocean-covered planets with hydrogen-rich atmospheres – which are more numerous and observable than Earth-like planets.

The researchers say the results, reported in The Astrophysical Journal, could mean that finding biosignatures of life outside our Solar System within the next few years is a real possibility.

“Hycean planets open a whole new avenue in our search for life elsewhere,” said Dr Nikku Madhusudhan from Cambridge’s Institute of Astronomy, who led the research.

Many of the prime Hycean candidates identified by the researchers are bigger and hotter than Earth, but still have the characteristics to host large oceans that could support microbial life similar to that found in some of Earth’s most extreme aquatic environments.

These planets also allow for a far wider habitable zone, or ‘Goldilocks zone’, compared to Earth-like planets. This means that they could still support life even though they lie outside the range where a planet similar to Earth would need to be in order to be habitable.

Thousands of planets outside our Solar System have been discovered since the first exoplanet was identified nearly 30 years ago. The vast majority are planets between the sizes of Earth and Neptune and are often referred to as ‘super-Earths’ or ‘mini-Neptunes’: they can be predominantly rocky or ice giants with hydrogen-rich atmospheres, or something in between.

Most mini-Neptunes are over 1.6 times the size of Earth: smaller than Neptune but too big to have rocky interiors like Earth. Earlier studies of such planets have found that the pressure and temperature beneath their hydrogen-rich atmospheres would be too high to support life.

However, a recent study on the mini-Neptune K2-18b by Madhusudhan’s team found that in certain conditions these planets could support life. The result led to a detailed investigation into the full range of planetary and stellar properties for which these conditions are possible, which known exoplanets may satisfy those conditions, and whether their biosignatures may be observable.

The investigation led the researchers to identify a new class of planets, Hycean planets, with massive planet-wide oceans beneath hydrogen-rich atmospheres. Hycean planets can be up to 2.6 times larger than Earth and have atmospheric temperatures up to nearly 200 degrees Celsius, depending on their host stars, but their oceanic conditions could be similar to those conducive for microbial life in Earth’s oceans. Such planets also include tidally locked ‘dark’ Hycean worlds that may have habitable conditions only on their permanent night sides, and ‘cold’ Hycean worlds that receive little radiation from their stars.

Planets of this size dominate the known exoplanet population, although they have not been studied in nearly as much detail as super-Earths. Hycean worlds are likely quite common, meaning that the most promising places to look for life elsewhere in the Galaxy may have been hiding in plain sight.

However, size alone is not enough to confirm whether a planet is Hycean: other aspects such as mass, temperature and atmospheric properties are required for confirmation.

When trying to determine what the conditions are like on a planet many light years away, astronomers first need to determine whether the planet lies in the habitable zone of its star, and then look for molecular signatures to infer the planet’s atmospheric and internal structure, which govern the surface conditions, presence of oceans and potential for life.

Astronomers also look for certain biosignatures which could indicate the possibility of life. Most often, these are oxygen, ozone, methane and nitrous oxide, which are all present on Earth. There are also a number of other biomarkers, such as methyl chloride and dimethyl sulphide, that are less abundant on Earth but can be promising indicators of life on planets with hydrogen-rich atmospheres where oxygen or ozone may not be as abundant.

“Essentially, when we’ve been looking for these various molecular signatures, we have been focusing on planets similar to Earth, which is a reasonable place to start,” said Madhusudhan. “But we think Hycean planets offer a better chance of finding several trace biosignatures.”

“It's exciting that habitable conditions could exist on planets so different from Earth,” said co-author Anjali Piette, also from Cambridge.

Madhusudhan and his team found that a number of trace terrestrial biomarkers expected to be present in Hycean atmospheres would be readily detectable with spectroscopic observations in the near future. The larger sizes, higher temperatures and hydrogen-rich atmospheres of Hycean planets make their atmospheric signatures much more detectable than Earth-like planets.

The Cambridge team identified a sizeable sample of potential Hycean worlds which are prime candidates for detailed study with next-generation telescopes, such as the James Webb Space Telescope (JWST), which is due to be launched later this year. These planets all orbit red dwarf stars between 35-150 light years away: close by astronomical standards. Already planned JWST observations of the most promising candidate, K2-18b, could lead to the detection of one or more biosignature molecules.

“A biosignature detection would transform our understanding of life in the universe,” said Madhusudhan. “We need to be open about where we expect to find life and what form that life could take, as nature continues to surprise us in often unimaginable ways.”

Reference:

Nikku Madhusudhan, Anjali A. A. Piette, and Savvas Constantinou. ‘Habitability and Biosignatures of Hycean Worlds.’ The Astrophysical Journal (2021). DOI: 10.3847/1538-4357/abfd9c

(The paper can also be viewed on arXiv.)

Source: University of Cambridge/Research News

---

Hubble Finds Water Vapor on Habitable-Zone Exoplanet for the First Time

PR Image heic1916a

Exoplanet K2-18b (Artist’s Impression)

---

Videos

PR Video heic1916a

Hubblecast Light: Exoplanet K2-18b

PR Video heic1916b

Animation of Exoplanet K2-18b (Artist’s Impression)

---

With data from the NASA/ESA Hubble Space Telescope, water vapour has been detected in the atmosphere of a super-Earth within the habitable zone by University College London (UCL) researchers in a world first. K2-18b, which is eight times the mass of Earth, is now the only planet orbiting a star outside the Solar System, or exoplanet, known to have both water and temperatures that could support life.

The discovery, published today in Nature Astronomy, is the first successful atmospheric detection of an exoplanet orbiting in its star’s habitable zone, at a distance where water can exist in liquid form.

First author, Dr Angelos Tsiaras (UCL Centre for Space Exochemistry Data,CSED), said: “Finding water on a potentially habitable world other than Earth is incredibly exciting. K2-18b is not ‘Earth 2.0’ as it is significantly heavier and has a different atmospheric composition. However, it brings us closer to answering the fundamental question: Is the Earth unique?”

The team used archive data from 2016 and 2017 captured by the NASA/ESA Hubble Space Telescope and developed open-source algorithms to analyse the starlight filtered through K2-18b’s atmosphere [1]. The results revealed the molecular signature of water vapour, also indicating the presence of hydrogen and helium in the planet’s atmosphere.

The authors believe that other molecules, including nitrogen and methane, may be present but they remain undetectable with current observations. Further studies are required to estimate cloud coverage and the percentage of atmospheric water present.

The planet orbits the cool dwarf star K2-18, which is 110 light years from Earth in the constellation of Leo. Given the high level of activity of its red dwarf star, K2-18b may be more hostile than Earth and is likely to be exposed to more radiation.

K2-18b was discovered in 2015 and is one of hundreds of super-Earths — planets with masses between those of Earth and Neptune — found by NASA’s Kepler spacecraft. NASA’s TESS mission is expected to detect hundreds more super-Earths in the coming years.

Co-author Dr Ingo Waldmann (UCL CSED), said: “With so many new super-Earths expected to be found over the next couple of decades, it is likely that this is the first discovery of many potentially habitable planets. This is not only because super-Earths like K2-18b are the most common planets in our Milky Way, but also because red dwarfs — stars smaller than our Sun — are the most common stars.”

The next generation of space telescopes, including the NASA/ESA/CSA James Webb Space Telescope and ESA’s ARIEL mission, will be able to characterise atmospheres in more detail as they will carry more advanced instruments. ARIEL is expected to launch in 2028 and will observe 1,000 planets in detail to get a truly representative picture of what they are like.

Professor Giovanna Tinetti (UCL CSED), co-author and Principal Investigator for ARIEL, said: “Our discovery makes K2-18b one of the most interesting targets for future study. Over 4000 exoplanets have been detected but we don’t know much about their composition and nature. By observing a large sample of planets, we hope to reveal secrets about their chemistry, formation and evolution.”

“This study contributes to our understanding of habitable worlds beyond our Solar System and marks a new era in exoplanet research, crucial to ultimately placing the Earth, our only home, into the greater picture of the Cosmos,” said Dr Tsiaras.

---

Notes

[1] The observations were achieved from 9 transits of K2-18b with the NASA/ESA Hubble Space Telescope’s Wide Field Camera 3 (WFC3), as part of the HST proposals 13665 and 14682 (PI: Björn Benneke).

---

More information

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.

Angelos Tsiaras, Ingo P. Waldmann, Giovanna Tinetti, Jonathan Tennyson & Sergey N. Yurchenko, ‘Water vapour in the atmosphere of the habitable-zone eight Earth-mass planet K2-18 b’ has been published in Nature Astronomy.

The research was funded by European Research Council and the UK Science and Technology Facilities Council which is part of UKRI. Image credit: ESA/Hubble, M. Kornmesser

---

Links

• Images of Hubble
• UCL release
• NASA release
• SpaceScoop article
• Science paper

---

Contacts

Dr. Angelos Tsiaras
UCL CSED
Cell: +44 (0)7477834386
Email: atsiaras@star.ucl.ac.uk

Dr. Ingo Waldmann
UCL CSED
Cell: +44 (0)7896320454
Email: ingo.waldmann@ucl.ac.uk

Dr. Rebecca Caygill
UCL Media Relations
Tel: +44 (0)20 3108 3846
Cell: +44 (0)7733307596
Email: r.caygill@ucl.ac.uk

Bethany Downer
ESA/Hubble, Public Information Officer
Garching, Germany
Email: bethany.downer@partner.eso.org

Source: ESA/Hubble/News

---