Scientists report the existence of 15 new planets -- including one 'super-Earth' that could harbor liquid water -- orbiting small, cool stars near our solar system. These stars, known as red dwarfs[1], are of enormous interest for studies of planetary formation and evolution.

A research team led by Teruyuki Hirano of Tokyo Institute of Technology's Department of Earth and Planetary Sciences has validated 15 exoplanets[2] orbiting red dwarf systems.

One of the brightest red dwarfs, K2-155 that is around 200 light years away from Earth, has three transiting super-Earths, which are slightly bigger than our own planet. Of those three super-Earths, the outermost planet, K2-155d, with a radius 1.6 times that of Earth, could be within the host star's habitable zone[3].

The findings, published in the form of two papers in The Astronomical Journal, are based on data from NASA Kepler spacecraft's second mission, K2, and follow-up observations using ground-based telescopes, including the Subaru Telescope in Hawaii and the Nordic Optical Telescope (NOT) in Spain.

The researchers found that K2-155d could potentially have liquid water on its surface based on three-dimensional global climate simulations. Hirano expresses both excitement and restraint, as he says: "In our simulations, the atmosphere and the composition of the planet were assumed to be Earth-like, and there's no guarantee that this is the case."

A more precise estimate of the radius and temperature of the K2-155 star would be needed to conclude definitively whether K2-155d is habitable. Achieving such precision would require further studies, for example, using interferometric techniques.

A key outcome from the current studies was that planets orbiting red dwarfs may have remarkably similar characteristics to planets orbiting solar-type stars.

"It's important to note that the number of planets around red dwarfs is much smaller than the number around solar-type stars," says Hirano. "Red dwarf systems, especially coolest red dwarfs, are just beginning to be investigated, so they are very exciting targets for future exoplanet research."

For example, while the so-called radius gap[4] of planets around solar-type stars has been reported previously, this is the first time that researchers have shown a similar gap in planets around red dwarfs. "This is a unique finding, and many theoretical astronomers are now investigating what causes this gap," says Hirano.

He adds that the most likely explanation for the lack of large planets in the proximity of host stars is photoevaporation, which can strip away the envelope of the planetary atmosphere.

The researchers also investigated the relationship between planet radius and metallicity[5] of the host star. "Large planets are only discovered around metal-rich stars," Hirano says, "and what we found was consistent with our predictions. The few planets with a radius about three times that of Earth were found orbiting the most metal-rich red dwarfs."

The studies were conducted as part of the KESPRINT collaboration, a group formed by the merger of KEST (Kepler Exoplanet Science Team) and ESPRINT (Equipo de Seguimiento de Planetas Rocosos Intepretando sus Transitos) in 2016.

With the planned launch of NASA's Transiting Exoplanet Survey Satellite (TESS) in April 2018, Hirano is hopeful that even more planets will be discovered. "TESS is expected to find many candidate planets around bright stars closer to Earth," he says. "This will greatly facilitate follow-up observations, including investigation of planetary atmospheres and determining the precise orbit of the planets."

Technical terms

[1] Red dwarfs: Also known as low-mass stars, red dwarfs are small, cool stars with a surface temperature of less than about 4,000 Kelvin.

[2] Exoplanets: Planets that orbit stars outside our solar system.

[3] Habitable zone: A region that is neither too hot nor too cold for liquid water, and hence life, to exist.

[4] Radius gap: Here referring to a reduced occurrence of planets that have a radius of between 1.5 to 2.0 times that of Earth.

[5] Metallicity: The amount of metal found in a star.</s>Researchers have discovered 15 new planets around cool stars known as red dwarfs, and one of those planets might be able to host liquid water on its surface.

In a new study, the researchers combined space telescope data with follow-up ground investigation to observe several red dwarfs, which are dim stars that weigh 7.5 to 50 percent the mass of the sun. The team spotted three exoplanets, each slightly larger than Earth, around red dwarf star K2-155, located 200 light-years away. The outermost planet of the system was a particularly interesting finding, as it had a radius about 1.6 times that of Earth and may be within this star's habitable zone, the researchers said. A star's habitable zone is the region where orbiting planets could be the right temperature to retain liquid water.

The team used observations from the NASA Kepler spacecraft's second mission, K2, and ground-based telescopes such as the Subaru Telescope in Hawaii and the Nordic Optical Telescope (NOT) in Spain to gather data on the 15 exoplanets, according to a recent statement about the research. To determine if K2-155's outermost planet, K2-155d, could have water on its surface, the team modeled what's known about this world in a three-dimensional climate simulation.

They found that this exoplanet may have liquid water. But despite the exciting possibilities, the researchers remain cautious until further analysis can confirm the initial findings, according to the statement.

"Red dwarf systems, especially coolest red dwarfs, are just beginning to be investigated, so they are very exciting targets for future exoplanet research," said Teruyuki Hirano, a researcher at the Tokyo Institute of Technology and lead author of the new work.

In April 2018, the NASA mission Transiting Exoplanet Survey Satellite (TESS)is scheduled to launch; that spacecraft may provide greater precision for researchers in search of distant planets with conditions that could support life. "TESS is expected to find many candidate planets around bright stars closer to Earth," Hirano said. "This will greatly facilitate follow-up observations, including investigation of planetary atmospheres and determining the precise orbit of the planets."

The new work was detailed Feb. 23 in The Astronomical Journal.</s>The TRAPPIST-1 system seems at first to be a good place to look for extraterrestrial life. It has seven rocky planets roughly the same size as Earth, and some of them are in the “habitable zone” of the star where liquid water could exist on the surface. Now, researchers from School of Earth and Space Exploration at Arizona State University say there might actually be too much water for the TRAPPIST-1 planets to harbor life.

Astronomers first announced the discovery of three exoplanets around TRAPPIST-1 in 2016. Then, a year later we learned of four more exoplanets in the system. TRAPPIST-1 is only 39 light years away, making it an excellent way to study the behavior and conditions on exoplanets. While all the planets are similar in size to Earth (no gas giants have been detected), only three of the planets (TRAPPIST-1e, f, and g) are orbiting at a distance that would allow them to have liquid water. Having liquid water is a requirement for life as we know it, but it turns out you might want some land, too.

All the TRAPPIST-1 planets were discovered with the transit method. A telescope watches a star for small dips in brightness. These dips can give away an exoplanet passing in front of the star from our perspective. That can tell us how large an exoplanet is. By watching the way transit signals vary over time, astronomers can also estimate an exoplanet’s mass. Put those together, and you’ve got an approximate density.

The Arizona State University used this data to create computer models of six of the seven planets in the TRAPPIST-1 system. They didn’t analyze the outermost planet, TRAPPIST-1h, because not enough is known about its properties. The innermost planets in the system (b and c) are believed to be about 10 percent water by mass. The more distant TRAPPIST-1f and g are a whopping 50 percent water. Exoplanets TRAPPIST-1d and e are in the middle of the system and have water masses in between the others.

You’ve probably heard Earth is 70 percent water, but that’s surface area. Water makes up just 0.2 percent of Earth’s mass. So, the TRAPPIST-1 planets could be incredibly wet. The outer planets would have more than 1,000 times the volume of water we have on Earth. The researchers point out this could impede the development of life because there are certain chemical processes that occur on dry land. In addition, the pressure of all that water pressing down on the mantle could prevent most volcanic activity. Without the carbon dioxide from volcanic activity, even planets in the habitable zone could have fallen victim to a runaway snowball effect.

Since TRAPPIST-1 is a cool red dwarf, all these planets orbit very close — less than the distance of Mercury’s orbit around the sun. That means they’re exposed to more radiation and solar flares. They’re probably also tidally locked so the same face always points toward the star. Having a lot of water could help disperse the heat and absorb radiation, so maybe there’s still some hope.</s>
• NASA's Kepler space telescope, which first launched in 2009 with the goal of finding new exoplanets, is running out of fuel 94 million miles from Earth's orbit.
• The NASA spacecraft has discovered thousands of worlds around other stars in our galaxy, with some of those worlds within a habitable range, where they might have liquid water.
• But though Kepler has lasted longer than some thought it would, every mission comes to an end, and Kepler will soon be pointed back home.

NASA's Kepler space telescope has discovered at least 2,342 exoplanets (worlds orbiting other stars) along with another 2,245 potential other worlds. At least 30 of those planets are less than twice Earth's size and in what we think could be the habitable zone around their stars.

Yet Kepler's search for other worlds is coming to an end, 94 million miles from home.

"With nary a gas station to be found in deep space, the spacecraft is going to run out of fuel," Charlie Sobeck, system engineer for the Kepler space telescope mission, wrote in a news release for NASA. "We expect to reach that moment within several months."

Kepler was first launched in 2009 with a mission to survey our part of the Milky Way galaxy, searching for Earth-size and smaller planets that might potentially be in a habitable zone so we can see just how common these sorts of planets are.

The spacecraft itself is a telescope that observes faraway stars to look for dips in brightness, which indicate a planet has passed in front of the star. The observations can be used to calculate how large the planet is and how far from a star it is — which reveals whether or not there could be liquid water, making a world habitable.

In 2013, a broken component put an end to Kepler's primary mission. Yet researchers were able to devise another way to observe distant worlds. The same spacecraft was reborn as "K2," and in that form has discovered at least 307 confirmed exoplanets and another 479 candidates.

At the time, NASA researchers thought they'd be able to conduct 10 observation campaigns before running out of gas. Kepler surprised them and is currently in its 17th campaign.

They can't tell exactly when the craft will run out of fuel, but the end is near. NASA plans on using a last bit of energy to point Kepler back towards Earth to ensure it can gather as much data as possible.

As this mission comes to an end, a replacement will come online. The Transiting Exoplanet Survey Satellite is scheduled to launch from Cape Canaveral, Florida, on April 16.

"TESS will search nearly the entire sky for planets outside our solar system, focusing on the brightest stars less than 300 light-years away, and adding to Kepler’s treasure trove of planet discoveries," wrote Sobeck.</s>Water is widely considered to be one of the key ingredients of life. However, in the case of exoplanets such as those orbiting the ultra-cool red dwarf star Trappist-1, the presence of water in abundant amounts could likely be detrimental to the appearance or sustenance of alien life.

The discovery of Trappist-1, which is slightly bigger than Jupiter and is located in the Aquarius constellation, around 40 light years away from our sun, excited researchers, especially after seven exoplanets were spotted orbiting the star. Researchers have yet to find a planetary system with such a large number of exoplanets.

A team of interdisciplinary researchers, studying the Trappist-1 planet’s habitability, discovered new details about its composition. The research team, consisting of geoscientists and astrophysicists from the Arizona State University (ASU) and Vanderbilt University, found that the planets have an abundant low density component.

Although in the case of most other alien worlds, this low density component would consist of atmospheric gases, researchers believe that in the case of Trappist-1 planets, this component is likely water.

"TRAPPIST-1 planets are too small in mass to hold onto enough gas to make up the density deficit,” geoscientist Cayman Unterborn said in a statement. “Even if they were able to hold onto the gas, the amount needed to make up the density deficit would make the planet much puffier than we see.”

In order to ascertain how much water is present on the Trappist-1 planets, the researchers used a custom-developed software, created by Unterborn and fellow researcher Alejandro Lorenzo. The software, dubbed ExoPlex, makes use of state-of-the-art mineral physics calculators and allowed the researchers to combine all of the available data on the Trappist-1 system.

This data, which includes information relating to the chemical make-up of the star and more, was harvested from a data repository called the Hypatia Catalogue. The database was formed by the new study’s co-author Natalie Hinkel. It consists of information on the stellar abundances of stars close to our sun, derived from over 150 literary sources.

The scientists discovered that the comparatively “dry” Trappist-1 exoplanets, located close to the host star were found to contain less than 15 percent water by mass. However, the outer planets in the planetary system were found to have over 50 percent water by mass. To put this in perspective, Earth has 0.02 percent water by mass. In other words, these alien planets have enough water to make up hundreds of Earth-oceans.

“What we are seeing for the first time are Earth-sized planets that have a lot of water or ice on them,” said Steven Desch, an astrophysicist at ASU and co-author of the new study.

Researchers also discovered that the Trappist-1 planets are actually much closer to the host star than the “ice line”. In every solar system, the ice line is the distance from the star, past which water exists as ice and can be accreted into the planet. However, within the ice line, water exists as vapor and won’t be accreted. This allowed researchers to determine that the Trappist-1 planets likely formed far away from their host star and later migrated closer, before settling in to their current orbits.

“The earlier the planets formed,” Desch said, “the farther away from the star they needed to have formed to have so much ice.”

The Trappist-1 exoplanets are about the same size as Earth and are terrestrial in nature, which ideally makes them potentially optimal sources of possible habitability. Previous studies have indicated the presence of water in these alien worlds, which has led to speculation about whether these planets could host alien life.

However, the findings of the new research indicate that despite water’s life-giving and supporting qualities, an overabundance of it on Trappist-1 exoplanets could make it highly unlikely for scientists to find any evidence of extraterrestrial life.

“We typically think having liquid water on a planet as a way to start life, since life, as we know it on Earth, is composed mostly of water and requires it to live,” Hinkel said. “However, a planet that is a water world, or one that doesn't have any surface above the water, does not have the important geochemical or elemental cycles that are absolutely necessary for life.”

Although there may be no alien life on the Trappist-1 planets, scientists still hope to be able to study them further to better understand how such icy worlds are formed and what kind of planets could be better candidates in our search for life in space.

The new study was published in the journal Nature Astronomy.
Fifteen new exoplanets are discovered. Three of them, larger than Earth, are also discovered around the dwarf star K2-155. Furthermore, a 3D climate simulation was created to find out if K2-155d has water.