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

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Goldilocks Stars Are Best Places to Look for Life

Comparison of G, K, and M Stars for Habitability

Credits: NASA, ESA, and Z. Levy (STScI) 

Science: NASA , ESA , and E. Guinan (Villanova University)

Release Images

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Orange Dwarf Stars Most Likely to Host Planets

To date astronomers have discovered over 4,000 planets orbiting other stars. Statistically, there should be over 100 billion planets in our Milky Way galaxy. They come in a wide range of sizes and characteristics, largely unimagined before exoplanets were first discovered in the mid-1990s. The biggest motivation for perusing these worlds is to find "Genesis II," a planet where life has arisen and evolved beyond microbes. The ultimate payoff would be finding intelligent life off the Earth.

A major step in searching for habitable planets is finding suitable stars that could foster the emergence of complex organisms. Because our Sun has nurtured life on Earth for nearly 4 billion years, conventional wisdom would suggest that stars like it would be prime candidates. But stars like our Sun represent only about 10% of the Milky Way population. What's more, they are comparatively short-lived. Our Sun is halfway through its estimated 10 billion-year lifetime.

Complex organisms arose on Earth only 500 million years ago. And, the modern form of humans has been here only for the blink of an eye on cosmological timescales: 200,000 years. The future of humanity is unknown. But what is for certain is that Earth will become uninhabitable for higher forms of life in a little over 1 billion years, as the Sun grows warmer and desiccates our planet.

Therefore, stars slightly cooler than our Sun — called orange dwarfs — are considered better hang-outs for advanced life. They can burn steadily for tens of billions of years. This opens up a vast timescape for biological evolution to pursue an infinity of experiments for yielding robust life forms. And, for every star like our Sun there are three times as many orange dwarfs in the Milky Way.

The only type of star that is more abundant are red dwarfs. But these are feisty little stars. They are so magnetically active they pump out 500 times as much radiation in the form of X-rays and ultraviolet light as our Sun does. Planets around these stars take a beating. They would be no place to call home for organisms like us.

An emerging idea, bolstered by stellar surveys performed by Hubble and other telescopes, is that the orange dwarfs are "Goldilocks stars" — not too hot, not too cool, and above all, not too violent to host life-friendly planets over a vast horizon of cosmic time.

In the search for life beyond Earth, astronomers look for planets in a star's "habitable zone" — sometimes nicknamed the "Goldilocks zone" — where temperatures are just right for liquid water to exist on a planet's surface to nurture life as we know it.

An emerging idea, bolstered by a three-decade-long set of stellar surveys, is that there are "Goldilocks stars" — not too hot, not too cool, and above all, not too violent to host life-friendly planets.

Because our Sun has nurtured life on Earth for nearly 4 billion years, conventional wisdom would suggest that stars like it would be prime candidates in the search for other potentially habitable worlds. In reality, stars slightly cooler and less luminous than our Sun, classified as K dwarfs, are the true "Goldilocks stars," said Edward Guinan of Villanova University, Villanova, Pennsylvania. "K-dwarf stars are in the 'sweet spot,' with properties intermediate between the rarer, more luminous, but shorter-lived solar-type stars (G stars) and the more numerous red dwarf stars (M stars). The K stars, especially the warmer ones, have the best of all worlds. If you are looking for planets with habitability, the abundance of K stars pump up your chances of finding life."

For starters, there are three times as many K dwarfs in our galaxy as stars like our Sun. Roughly 1,000 K stars lie within 100 light-years of our Sun as prime candidates for exploration. These so-called orange dwarfs live from 15 billion to 45 billion years. By contrast, our Sun, now already halfway through its lifetime, lasts for only 10 billion years. Its comparatively rapid rate of stellar evolution will leave the Earth largely uninhabitable in just another 1 or 2 billion years. "Solar-type stars limit how long a planet's atmosphere can remain stable," Guinan said. That's because a billion or so years from now, Earth will orbit inside the hotter (inner) edge of the Sun's habitable zone, which moves outward as the Sun grows warmer and brighter. As a result, the Earth will be desiccated as it loses its present atmosphere and oceans. By an age of 9 billion years the Sun will have swelled up to become a red giant that could engulf the Earth.

Despite their small size, the even more abundant red dwarf stars, also known as M dwarf stars, have even longer lifetimes and appear to be hostile to life as we know it. Planets that are located in a red dwarf's comparatively narrow habitable zone, which is very close to the star, are exposed to extreme levels of X-ray and ultraviolet (UV) radiation, which can be up to hundreds of thousands of times more intense than what Earth receives from the Sun. A relentless fireworks show of flares and coronal mass ejections bombard planets with a dragon's breath of seething plasma and showers of penetrating high-energy particles. Red dwarf habitable-zone planets can be baked bone dry and have their atmospheres stripped away very early in their lives. This could likely prohibit the planets from evolving to be more hospitable a few billion years after red dwarf outbursts have subsided. "We're not so optimistic anymore about the chances of finding advanced life around many M stars," Guinan said.

The K dwarfs do not have intensely active magnetic fields that power strong X-ray and UV emissions and energetic outbursts, and therefore they shoot off flares much less frequently, based on Guinan's research. Accompanying planets would get about 1/100th as much deadly X-ray radiation as those orbiting the close-in habitable zones of magnetically-active M stars.

In a program called the "GoldiloKs" Project, Guinan and his Villanova colleague Scott Engle, are working with undergraduate students to measure the age, rotation rate, and X-ray and far-ultraviolet radiation in a sampling of mostly cool G and K stars.They are using NASA's Hubble Space Telescope, Chandra X-ray Observatory, and the European Space Agency's XMM-Newton satellite for their observations. Hubble's sensitive ultraviolet-light observations of radiation from hydrogen were used to assess the radiation from a sample of about 20 orange dwarfs. "Hubble is the only telescope that can do this kind of observation," Guinan said.

Guinan and Engle found that the levels of radiation were much more benign to any accompanying planets than those found around red dwarfs. K stars also have longer lifetimes and therefore slower migration of the habitable zone. Therefore, K dwarfs seem like the ideal place to go looking for life, and these stars would allow time for highly evolved life to develop on planets. Over the Sun's entire lifetime — 10 billion years — K stars only increase their brightness by about 10-15%, giving biological evolution a much longer timespan to evolve advanced life forms than on Earth.

Guinan and Engle looked at some of the more interesting K stars hosting planets, including Kepler-442, Tau Ceti, and Epsilon Eridani. (The latter two were early targets of the late 1950s Project Ozma — the first attempt to detect radio transmissions from extraterrestrial civilizations.)

"Kepler-442 is noteworthy in that this star (spectral classification, K5) hosts what is considered one of the best Goldilocks planets, Kepler-442b, a rocky planet that is a little more than twice Earth's mass. So the Kepler-442 system is a Goldilocks planet hosted by a Goldilocks star!" said Guinan.

Over the last 30 years Guinan and Engle and their students have observed a variety of stellar types. Based on their studies, the researchers have determined relationships among stellar age, rotation rate, X-ray-UV emissions and flare activity. These data have been utilized to investigate the effects of high-energy radiation on planet atmospheres and possible life.

The results are being presented at the 235th meeting of the American Astronomical Society in Honolulu, Hawaii.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

Source: HubbleSite/News

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Contact:

Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4514
villard@stsci.edu

Edward Guinan
Villanova University, Villanova, Pennsylvania
edward.guinan@villanova.edu

Related Links: NASA's Hubble Portal

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Eight New Planets Found in "Goldilocks" Zone

This artist’s conception depicts an Earth-like planet orbiting an evolved star that has formed a stunning "planetary nebula." Earlier in its life, this planet may have been like one of the eight newly discovered worlds orbiting in the habitable zones of their stars. Credit: David A. Aguilar (CfA). High Resolution (jpg) - Low Resolution (jpg)

Cambridge, MA -Astronomers announced today that they have found eight new planets in the "Goldilocks" zone of their stars, orbiting at a distance where liquid water can exist on the planet's surface. This doubles the number of small planets (less than twice the diameter of Earth) believed to be in the habitable zone of their parent stars. Among these eight, the team identified two that are the most similar to Earth of any known exoplanets to date.

"Most of these planets have a good chance of being rocky, like Earth," says lead author Guillermo Torres of the Harvard-Smithsonian Center for Astrophysics (CfA).

These findings were announced today in a press conference at a meeting of the American Astronomical Society.

The two most Earth-like planets of the group are Kepler-438b and Kepler-442b. Both orbit red dwarf stars that are smaller and cooler than our Sun. Kepler-438b circles its star every 35 days, while Kepler-442b completes one orbit every 112 days.

With a diameter just 12 percent bigger than Earth, Kepler-438b has a 70-percent chance of being rocky, according to the team's calculations. Kepler-442b is about one-third larger than Earth, but still has a 60-percent chance of being rocky.

To be in the habitable zone, an exoplanet must receive about as much sunlight as Earth. Too much, and any water would boil away as steam. Too little, and water will freeze solid.

"For our calculations we chose to adopt the broadest possible limits that can plausibly lead to suitable conditions for life," says Torres.

Kepler-438b receives about 40 percent more light than Earth. (In comparison, Venus gets twice as much solar radiation as Earth.) As a result, the team calculates it has a 70 percent likelihood of being in the habitable zone of its star.

Kepler-442b get about two-thirds as much light as Earth. The scientists give it a 97 percent chance of being in the habitable zone.

"We don't know for sure whether any of the planets in our sample are truly habitable," explains second author David Kipping of the CfA. "All we can say is that they're promising candidates."

Prior to this, the two most Earth-like planets known were Kepler-186f, which is 1.1 times the size of Earth and receives 32 percent as much light, and Kepler-62f, which is 1.4 times the size of Earth and gets 41 percent as much light.

The team studied planetary candidates first identified by NASA's Kepler mission. All of the planets were too small to confirm by measuring their masses. Instead, the team validated them by using a computer program called BLENDER to determine that they are statistically likely to be planets. BLENDER was developed by Torres and colleague Francois Fressin, and runs on the Pleaides supercomputer at NASA Ames. This is the same method that has been used previously to validate some of Kepler's most iconic finds, including the first two Earth-size planets around a Sun-like star and the first exoplanet smaller than Mercury.

After the BLENDER analysis, the team spent another year gathering follow-up observations in the form of high-resolution spectroscopy, adaptive optics imaging, and speckle interferometry to thoroughly characterize the systems.

Those follow-up observations also revealed that four of the newly validated planets are in multiple-star systems. However, the companion stars are distant and don't significantly influence the planets.

As with many Kepler discoveries, the newly found planets are distant enough to make additional observations challenging. Kepler-438b is located 470 light-years from Earth while the more distant Kepler-442b is 1,100 light-years away.

The paper reporting these results has been accepted for publication in The Astrophysical Journal and is available online.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu

Source:  Harvard-Smithsonian Center for Astrophysics

 

Carbon Worlds May be Waterless, Finds NASA Study

This artist's concept illustrates the fate of two different planets: the one on the left is similar to Earth, made up largely of silicate-based rocks with oceans coating its surface. The one on the right is rich in carbon -- and dry. Chances are low that life as we know it, which requires liquid water, would thrive under such barren conditions. Image credit: NASA/JPL-Caltech.  › Full image and caption

Planets rich in carbon, including so-called diamond planets, may lack oceans, according to NASA-funded theoretical research.

Our sun is a carbon-poor star, and as result, our planet Earth is made up largely of silicates, not carbon. Stars with much more carbon than the sun, on the other hand, are predicted to make planets chock full of carbon, and perhaps even layers of diamond.

By modeling the ingredients in these carbon-based planetary systems, the scientists determined they lack icy water reservoirs thought to supply planets with oceans.

"The building blocks that went into making our oceans are the icy asteroids and comets," said Torrence Johnson of NASA's Jet Propulsion Laboratory in Pasadena, Calif, who presented the results Oct. 7 at the American Astronomical Society Division of Planetary Sciences meeting in Denver. Johnson, a team member of several NASA planetary missions, including Galileo, Voyager and Cassini, has spent decades studying the planets in our own solar system.

"If we keep track of these building blocks, we find that planets around carbon-rich stars come up dry," he said.

Johnson and his colleagues say the extra carbon in developing star systems would snag the oxygen, preventing it from forming water.

"It's ironic that if carbon, the main element of life, becomes too abundant, it will steal away the oxygen that would have made water, the solvent essential to life as we know it," said Jonathan Lunine of Cornell University, Ithaca, N.Y., a collaborator on the research.

One of the big questions in the study of planets beyond our solar system, called exoplanets, is whether or not they are habitable. Researchers identify such planets by first looking for those that are situated within the "habitable zone" around their parent stars, which is where temperatures are warm enough for water to pool on the surface. NASA's Kepler mission has found several planets within this zone, and researchers continue to scrutinize the Kepler data for candidates as small as Earth.

But even if a planet is found in this so-called "Goldilocks" zone, where oceans could, in theory, abound, is there actually enough water available to wet the surface? Johnson and his team addressed this question with planetary models based on measurements of our sun's carbon-to-oxygen ratio. Our sun, like other stars, inherited a soup of elements from the Big Bang and from previous generations of stars, including hydrogen, helium, nitrogen, silicon, carbon and oxygen.

"Our universe has its own top 10 list of elements," said Johnson, referring to the 10 most abundant elements in our universe.

These models accurately predict how much water was locked up in the form of ice early in the history of our solar system, billions of years ago, before making its way to Earth. Comets and/or the parent bodies of asteroids are thought to have been the main water suppliers, though researchers still debate their roles. Either way, the objects are said to have begun their journey from far beyond Earth, past a boundary called the "snow line," before impacting Earth and depositing water deep in the planet and on its surface.

When the researchers applied the planetary models to the carbon-rich stars, the water disappeared. "There's no snow beyond the snow line," said Johnson.

"All rocky planets aren't created equal," said Lunine. "So-called diamond planets the size of Earth, if they exist, will look totally alien to us: lifeless, ocean-less desert worlds."

The computer model results supporting these conclusions were published in the Astrophysical Journal last year (http://arxiv.org/abs/1208.3289). The implications for habitability in these systems were the focus of the Division of Planetary Sciences meeting.

The California Institute of Technology, Pasadena, manages JPL for NASA.

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov  

FOUND: One Planet Orbiting Sun-like Star. Only Twelve Light Years Away. May Be Habitable.

Artist’s impression of the Tau Ceti system 

Credit: J. Pinfield for the RoPACS network at the University of Hertfordshire, 2012

Tau Ceti in the early evening sky in constellation of Cetus on Wednesday 19th December from Hatfield, UK

An international team of astronomers using the W. M. Keck Observatory and other telescopes, has discovered that Tau Ceti, one of the closest and most Sun-like stars, may host five planets – with one in the elusive ‘Goldilocks Zone’. Credit: Stellarium software

At a distance of twelve light years and visible with the naked eye in the December evening sky, Tau Ceti is the closest single star that has the same spectral classification as our Sun. Its five planets are estimated to have masses between two and six times the mass of the Earth – making it the lowest-mass planetary system yet detected. One of the planets lies in the star’s habitable zone – the so-called Goldilocks Zone with it’s ‘just right’ temperatures for supporting liquid water – and has a mass around five times that of Earth, making it the smallest planet found to be orbiting in the habitable zone of any Sun-like star. 

The international team of astronomers, from the UK, Chile, the USA, and Australia, combined more than 6,000 observations from three different instruments, including HIRES on the Keck I telescope. Using new techniques, the team has found a method to detect signals half the size previously thought possible. This greatly improves the sensitivity of searches for small planets and suggests that Tau Ceti is not a lone star but has a planetary system.

“We pioneered new data modeling techniques by adding artificial signals to the data and testing our recovery of the signals with a variety of different approaches,” said Mikko Tuomi from the University of Hertfordshire and the first author of the paper. “This significantly improved our noise modeling techniques and increased our ability to find low mass planets.”

“We chose Tau Ceti for this noise modeling study because we had thought it contained no signals,” said Hugh Jones from the University of Hertfordshire. “And as it is so bright and similar to our Sun, it is an ideal benchmark system to test out our methods for the detection of small planets.”

“Tau Ceti is one of our nearest cosmic neighbors and so bright that we may be able to study the atmospheres of these planets in the not too distant future,” said James Jenkins, Universidad de Chile and Visiting Fellow at the University of Hertfordshire. “Planetary systems found around nearby stars close to our Sun indicate that these systems are common in our Milky Way galaxy.”

More than 800 planets have been discovered orbiting other worlds, but planets in orbit around the nearest Sun-like stars are particularly interesting. “This discovery is in keeping with our emerging view that virtually every star has planets, and that the galaxy must have many such potentially habitable Earth-sized planets,” said Steve Vogt from University of California Santa Cruz. “They are everywhere, even right next door! We are now beginning to understand that nature seems to overwhelmingly prefer systems that have multiple planets with orbits of less than one hundred days. This is quite unlike our own solar system where there is nothing with an orbit inside that of Mercury. So our solar system is, in some sense, a bit of a freak and not the most typical kind of system that nature cooks up.”

“As we stare at the night sky, it is worth contemplating that there may well be more planets out there than there are stars – some fraction of which may well be habitable,” said Chris Tinney from the University of New South Wales.

The W. M. Keck Observatory operates Earth’s two biggest and most scientifically productive telescopes on the summit of Mauna Kea, Island of Hawaii. The twin, 10-meter telescopes feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and a world-leading laser guide star adaptive optics system. The Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.

Media Contact:

Steve Jefferson
Communications Officer, Advancement
W.M. Keck Observatory
sjefferson@keck.hawaii.edu
(808)881-3827

Science Contact:

Mikko Tuomi
University of Hertfordshire
miptuom@utu.fi

Hugh Jones
University of Hertfordshire
h.r.a.jones@herts.ac.uk