  The core accretion theory for giant planet formation predicts enrichment of
elemental abundances in planetary envelopes caused by runaway accretion of
planetesimals, which is consistent with measured super-solar abundances of C,
N, P, S, Xe, and Ar in Jupiter's atmosphere. However, the abundance of O which
is expected to be the most dominant constituent of planetesimals is unknown for
solar system giant planets, owing to the condensation of water in their
ultra-cold atmospheres, thereby posing a key unknown in solar system formation.
On the other hand, hundreds of extrasolar hot Jupiters are known with very high
temperatures (>~1000 K) making them excellent targets to measure H2O abundances
and, hence, oxygen in their atmospheres. We constrain the atmospheric H2O
abundances in three hot Jupiters (HD 189733b, HD 209458b, and WASP-12b),
spanning a wide temperature range (1200-2500 K), using their near-infrared
transmission spectra obtained using the HST WFC3 instrument. We report
conclusive measurements of H2O in HD 189733b and HD 209458b, while that in
WASP-12b is not well constrained by present data. The data allow nearly solar
as well as significantly sub-solar abundances in HD 189733b and WASP-12b.
However, for HD 209458b, we report the most precise H2O measurement in an
exoplanet to date that suggests a ~20-135 sub-solar H2O abundance. We discuss
the implications of our results on the formation conditions of hot Jupiters and
on the likelihood of clouds in their atmospheres. Our results highlight the
critical importance of high-precision spectra of hot Jupiters for deriving
their H2O abundances.
