A team led by researchers at the University of Montreal has found evidence that two exoplanets orbiting a red dwarf star are "water worlds," where water makes up a large fraction of the entire planet. These worlds, located in a planetary system 218 light-years away in the constellation Lyra, are unlike any planet found in our solar system.
The team, led by Caroline Piaulet of
the Trottier
Institute for Research on Exoplanets at the University of Montreal, published a detailed
study of this
planetary system, known as Kepler-138, in the journal Nature Astronomy today.
In this illustration super-Earth Kepler-138 d is in the foreground. To the left, the planet Kepler-138 c, and in the background the planet Kepler 138 b, seen in silhouette transiting its central star. Kepler 138 is a red dwarf star located 218 light-years away. The low density of Kepler-138 c and Kepler-138 d – which are nearly identical in size – means that they must be composed largely of water. They are both twice Earth's mass but have roughly half of Earth's density, and therefore cannot be solid rock. This is based on measurements of their mass versus physical diameter. They are considered a new class of "water planet," unlike any major planet found in our solar system. Kepler-138 b is one of the smallest exoplanets known, having the mass of the planet Mars and the density of rock. Credits: NASA, ESA, and Leah Hustak (STScI)
Piaulet and colleagues observed exoplanets Kepler-138 c and Kepler-138 d with NASA's Hubble and the retired Spitzer space telescopes and discovered that the planets could be composed largely of water. These two planets and a smaller planetary companion closer to the star, Kepler-138 b, had been discovered previously by NASA's Kepler Space Telescope. The new study found evidence for a fourth planet, too.
Water wasn't directly detected at Kepler-138 c and d, but by comparing the sizes and masses of the planets to models, astronomers conclude that a significant fraction of their volume – up to half of it – should be made of materials that are lighter than rock but heavier than hydrogen or helium (which constitute the bulk of gas giant planets like Jupiter). The most common of these candidate materials is water.
"We previously thought that planets that were a bit larger than Earth were big balls of metal and rock, like scaled-up versions of Earth, and that's why we called them super-Earths," explained Björn Benneke, study co-author and professor of astrophysics at the University of Montreal. "However, we have now shown that these two planets, Kepler-138 c and d, are quite different in nature and that a big fraction of their entire volume is likely composed of water. It is the best evidence yet for water worlds, a type of planet that was theorized by astronomers to exist for a long time."
This
is an artist's illustration showing a cross-section of the Earth (left) and the
exoplanet Kepler-138 d (right). Like the Earth, this exoplanet has an interior
composed of metals and rocks (brown portion), but Kepler-138 d also has a thick
layer of high-pressure water in various forms: supercritical and potentially
liquid water deep inside the planet and an extended water vapor envelope
(shades of blue) above it. These water layers make up more than 50% of its
volume, or a depth of about 1,243 miles (2,000 kilometers). The Earth, in
comparison, has a negligible fraction of liquid water with an average ocean
depth of less than 2.5 miles (4 kilometers). Credits: Benoit Gougeon
(University of Montreal)
With volumes more
than three times that of Earth and masses twice as big, planets c and d have
much lower densities than Earth. This is surprising because most of the planets
just slightly bigger than Earth that have been studied in detail so far all
seemed to be rocky worlds like ours. The closest comparison, say researchers,
would be some of the icy moons in the outer solar system that are also largely
composed of water surrounding a rocky core.
"Imagine larger versions of Europa or Enceladus, the
water-rich moons orbiting Jupiter and Saturn, but brought much closer to their
star," explained Piaulet. "Instead of an icy surface, they would
harbor large water-vapor envelopes."
Researchers caution the planets may not
have oceans like those on Earth directly at the planet's surface. "The
temperature in Kepler-138 d's atmosphere is likely above the boiling point of
water, and we expect a thick dense atmosphere made of steam on this planet.
Only under that steam atmosphere there could potentially be liquid water at
high pressure, or even water in another phase that occurs at high pressures,
called a supercritical fluid," Piaulet said.
In 2014, data from NASA's Kepler Space
Telescope allowed astronomers to announce the detection of three planets
orbiting Kepler-138. This was based on a measurable dip in starlight as the
planet momentarily passed in front of their star.
Benneke and his colleague Diana Dragomir, from the University of New Mexico, came up with the idea of re-observing the
planetary system with the Hubble and Spitzer space telescopes between 2014 and
2016 to catch more transits of Kepler-138 d, the third planet in the system, in
order to study its atmosphere.
A new exoplanet in the system
The two possible water worlds, Kepler-138
c and d, are not located in the habitable zone, the area around a star where
temperatures would allow liquid water on the surface of a rocky planet. But in
the Hubble and Spitzer data, researchers additionally found evidence for a new
planet in the system, Kepler-138 e, in the habitable zone.
This newly found planet is small and
farther from its star than the three others, taking 38 days to complete an
orbit. The nature of this additional planet, however, remains an open question
because it does not seem to transit its host star. Observing the exoplanet's
transit would have allowed astronomers to determine its size.
With Kepler-138 e now in the picture, the
masses of the previously known planets were measured again via the transit
timing-variation method, which consists of tracking small variations in the
precise moments of the planets' transits in front of their star caused by the
gravitational pull of other nearby planets.
The researchers had another surprise: they
found that the two water worlds Kepler-138 c and d are "twin"
planets, with virtually the same size and mass, while they were previously
thought to be drastically different. The closer-in planet, Kepler-138 b, on the
other hand, is confirmed to be a small Mars-mass planet, one of the smallest
exoplanets known to date.
"As our instruments and techniques become sensitive enough to find and study planets that are farther from their stars, we might start finding a lot more of these water worlds," Benneke concluded.
Using data from NASA’s Hubble and Spitzer Space Telescopes, astronomers found evidence that two exoplanets orbiting a star 218 light-years away are “water worlds,” where water makes up a large fraction of the entire planet. Credits: NASA Goddard Space Flight Center, Lead Producer: Paul Morris
The Hubble Space Telescope is a project of international cooperation between NASA and ESA. 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: Two Exoplanets May Be Mostly Water, NASA's Hubble and Spitzer Find | NASA
No comments:
Post a Comment