3810
- WATER PLANETS - new
discoveries lightyears away? 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. Maybe some identical to Earth.
--------- 3810
- WATER PLANETS
- new discoveries lightyears
away?
- Two of the 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.
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- This planetary system is known as
“Kepler-138”. These two 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.
( The small alphetical letters designate
planets. The name designinate their
sun.)
-
- 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.
-
- 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. 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.
-
- 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 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. 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”.
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- In 2014 the detection of three planets
orbiting Kepler-138 was based on a measurable dip in starlight as the planet
momentarily passed in front of their star.
-
- By re-observing the planetary system
with the Hubble and Spitzer space telescopes between 2014 and 2016 in order to
catch more transits of Kepler-138 d, the third planet in the system, in order
to study its atmosphere.
-
- 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, that was 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 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. Maybe some identical to Earth.
-
January 1, 2022 WATER PLANETS
- discoveries lightyears away? 3810
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