This is an artistic illustration of the gas giant
planet HD 209458b in the constellation Pegasus. To the surprise of astronomers,
they have found much less water vapor in the hot world’s atmosphere than
standard planet-formation models predict.
Image Credit:
NASA, ESA, G. Bacon (STScI) and N. Madhusudhan
(UC)
Astronomers using NASA's Hubble Space Telescope have gone looking for water
vapor in the atmospheres of three planets orbiting stars similar to the sun --
and have come up nearly dry.
The three planets, known as HD 189733b, HD 209458b, and WASP-12b, are between
60 and 900 light-years away from Earth and were thought to be ideal candidates
for detecting water vapor in their atmospheres because of their high
temperatures where water turns into a measurable vapor.
These so-called “hot Jupiters” are so close to their star they have
temperatures between 1,500 and 4,000 degrees Fahrenheit, however, the planets
were found to have only one-tenth to one one-thousandth the amount of water
predicted by standard planet-formation theories.
"Our water measurement in one of the planets, HD 209458b, is the
highest-precision measurement of any chemical compound in a planet outside our
solar system, and we can now say with much greater certainty than ever before
that we've found water in an exoplanet," said Nikku Madhusudhan of the Institute
of Astronomy at the University of Cambridge, England. "However, the low water
abundance we have found so far is quite astonishing."
Madhusudhan, who led the research, said that this finding presents a major
challenge to exoplanet theory. "It basically opens a whole can of worms in
planet formation. We expected all these planets to have lots of water in them.
We have to revisit planet formation and migration models of giant planets,
especially “hot Jupiters,” and investigate how they're formed."
He emphasizes that these results may have major implications in the search
for water in potentially habitable Earth-sized exoplanets. Instruments on future
space telescopes may need to be designed with a higher sensitivity if target
planets are drier than predicted. "We should be prepared for much lower water
abundances than predicted when looking at super-Earths (rocky planets that are
several times the mass of Earth)," Madhusudhan said.
Using near-infrared spectra of the planets observed with Hubble, Madhusudhan
and his collaborators estimated the amount of water vapor in each of the
planetary atmospheres that explains the data.
The planets were selected because they orbit relatively bright stars that
provide enough radiation for an infrared-light spectrum to be taken. Absorption
features from the water vapor in the planet's atmosphere are detected because
they are superimposed on the small amount of starlight that glances through the
planet's atmosphere.
Detecting water is almost impossible for transiting planets from the ground
because Earth's atmosphere has a lot of water in it, which contaminates the
observation. "We really need the Hubble Space Telescope to make such
observations," said Nicolas Crouzet of the Dunlap Institute at the University of
Toronto and co-author of the study.
The currently accepted theory on how giant planets in our solar system
formed, known as core accretion, states a planet is formed around the young star
in a protoplanetary disk made primarily of hydrogen, helium, and particles of
ices and dust composed of other chemical elements. The dust particles stick to
each other, eventually forming larger and larger grains. The gravitational
forces of the disk draw in these grains and larger particles until a solid core
forms. This then leads to runaway accretion of both solids and gas to eventually
form a giant planet.
This theory predicts that the proportions of the different elements in the
planet are enhanced relative to those in its star, especially oxygen, which is
supposed to be the most enhanced. Once the giant planet forms, its atmospheric
oxygen is expected to be largely encompassed within water molecules. The very
low levels of water vapor found by this research raise a number of questions
about the chemical ingredients that lead to planet formation.
"There are so many things we still don't know about exoplanets, so this opens
up a new chapter in understanding how planets and solar systems form," said
Drake Deming of the University of Maryland, who led one of the precursor
studies. "The problem is that we are assuming the water to be as abundant as in
our own solar system. What our study has shown is that water features could be a
lot weaker than our expectations."
The findings are published July 24 in The Astrophysical Journal Letters.
The Hubble Space Telescope is a project of international cooperation between
NASA and the European Space Agency. NASA's Goddard Space Flight Center in
Greenbelt, Maryland, manages the telescope. The Space Telescope Science
Institute (STScI) in Baltimore conducts Hubble science operations. STScI is
operated for NASA by the Association of Universities for Research in Astronomy,
Inc., in Washington.
For images and more information about Hubble, visit:
and
NASA
Guillermo Gonzalo Sánchez Achutegui
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