This is an artistic illustration of the gas giant planet HD 209458b
(unofficially named Osiris) located 150 light-years away in the
constellation Pegasus. This is a "hot Jupiter" class planet. Estimated
to be 220 times the mass of Earth. The planet's atmosphere is a
seething 2,150 degrees Fahrenheit. It orbits very closely to its bright
sunlike star, and the orbit is tilted edge-on to Earth. This makes the
planet an ideal candidate for the Hubble Space Telescope to be used to
make precise measurements of the chemical composition of the giant's
atmosphere as starlight filters though it. To the surprise of
astronomers, they have found much less water vapor in the atmosphere
than standard planet-formation models predict. Credit: NASA, ESA, and G. Bacon (STScI)
This graph compares observations with modeled infrared spectra of
three hot-Jupiter-class exoplanets that were spectroscopically observed
with the Hubble Space Telescope. The red curve in each case is the
best-fit model spectrum for the detection of water vapor absorption in
the planetary atmosphere. The blue circles and error bars show the
processed and analyzed data from Hubble's spectroscopic observations. Credit: NASA, ESA, N. Madhusudhan (University of Cambridge), and A. Feild and G. Bacon (STScI)
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, HD 189733b, HD 209458b, and WASP-12b, are between
60 and 900 light-years away. These giant gaseous worlds are so hot,
with temperatures between 1,500 and 4,000 degrees Fahrenheit, that they
are ideal candidates for detecting water vapor in their atmospheres.
However, to the surprise of the researchers, the planets surveyed
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 the solar system, and we can now say with much greater
certainty than ever before that we've found water in an exoplanet,"
said Dr. Nikku Madhusudhan of the Institute of Astronomy at the
University of Cambridge, United Kingdom, who led the research.
"However, the low water abundance we are finding is quite astonishing."
Madhusudhan 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, though found in these large hot
planets close to their parent stars, may have major implications for
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 from the Space Telescope Science
Institute, Baltimore, Maryland; the University of Maryland, College
Park, Maryland; the Johns Hopkins University, Baltimore, Maryland; and
the Dunlap Institute at the University of Toronto, Ontario, Canada,
estimated the amount of water vapor in the planetary atmospheres based
on sophisticated computer models and statistical techniques to explain
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 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 that
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 is known as core accretion, in which 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 core 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 their star, especially
oxygen that 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 raises a number of questions about the chemical
ingredients that lead to planet formation, say researchers.
"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 being published on July 24 in The Astrophysical Journal Letters.CONTACT
Ray VillardSpace Telescope Science Institute, Baltimore, Md.
410-338-4514
villard@stsci.edu
Nikku Madhusudhan
Institute of Astronomy, University of Cambridge, United Kingdom
617-475-5112 (or 011-44-01223-766619)
nmadhu@ast.cam.ac.uk
Source: HubbleSite
