Astronomers discover a near-grazing transiting ‘hot Jupiter’

The WASP discovery photometry (top) and follow-up transit
lightcurves (bottom). Credit: Temple et al., 2018.

An international group of astronomers has detected a new “hot
Jupiter” exoplanet making a near-grazing transit of its host
star. The newly found alien world, designated WASP-174b, is
most likely similar in size and mass to Jupiter, however much
hotter than our solar system’s biggest planet. The discovery
is reported February 2 in a paper published on

A grazing transit means that a planet only partially transits
its parent star’s disc. Such planets are important for
researchers, as they offer a great opportunity to find
additional bodies orbiting host . This is because other, yet undetected
objects would perturb the grazing planet’s orbit and could
potentially induce periodic variations of the transit impact
parameter, leading to transit duration variations. However,
although astronomers have identified hundreds of exoplanets,
only a handful of grazing extrasolar worlds have been
discovered to date.

Recently, a team of researchers led by Lorna Temple of the
Keele University, U.K., has found a new example of a grazing
exoplanet as part of the WASP-South transit survey. The
astronomers identified a transit signal in the light curve of
the star WASP-174 from the spectroscopic data gathered by the
WASP-South telescope at the South African Astronomical
Observatory (SAAO) in South Africa between May 2006 and June
2012. The planetary nature of this signal was later confirmed
by Doppler tomography method using the HARPS spectrograph
mounted on ESO’s 3.6-m telescope, along with photometric data
from the TRAPPIST South telescope and the SPECULOOS Southern
Observatory, all located in Chile.

“We report here the discovery of a hot Jupiter found as a
candidate in the WASP-South transit survey and confirmed by
Doppler tomography using the ESO 3.6-m/HARPS spectrograph,
together with follow-up photometry from the TRAPPISTSouth and
SPECULOOS Southern Observatory telescopes,” the researchers
wrote in the paper.

The authors noted that due to the grazing or near-grazing
of WASP-174b, the planet’s radius was
difficult to measure. They assume that the value of this
parameter lies between 0.7 to 1.7 Jupiter radii.

Furthermore, the researchers added that the mass of WASP-174b
is also uncertain, given that the parent star is hot (6,400 K)
and rotates rapidly (rotation period below 4.4 days), which
limits the accuracy and precision of radial-velocity
measurements. They only managed to calculate that the planet’s
mass should be no greater than 1.3 Jupiter masses.

The study found that WASP-174b orbits its star every 4.23 days
at a distance of about 0.0555 AU from the host. It is estimated
that the planet’s equilibrium temperature is 1,470 K.

According to the study, the derived parameters of WASP-174b
suggest that the exoplanet is a fairly inflated as it should
have a radius larger than expected in gas giant planets. It was
also classified as a “hot Jupiter,” given that it is similar in
characteristics to Jupiter, orbits its very closely, and therefore has a
high surface temperature.

The host is a 1.65-billion-year old star of spectral type F6V,
about 30 percent larger and more massive than the sun. It has a
V-band magnitude of 11.9, which, according to the authors of
the paper, makes WASP-174 the faintest “hot-Jupiter” system for
which the shadow of the planet has been detected by tomographic

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More information: Discovery of WASP-174b: Doppler
tomography of a near-grazing transit, arXiv:1802.00766

We report the discovery and tomographic detection of WASP-174b,
a planet with a near-grazing transit on a 4.23-d orbit around a
V = 11.9, F6V star with [Fe/H] = 0.09 ± 0.09. The planet is in
a moderately misaligned orbit with a sky-projected spin-orbit
angle of λ = 31 ± 1. This is in
agreement with the known tendency for orbits around hotter
stars to be misaligned. Owing to the grazing transit the
planet’s radius is uncertain, with a possible range of 0.7-1.7
RJup. The planet’s mass has an upper limit of 1.3 MJup.
WASP-174 is the faintest hot-Jupiter system so far confirmed by
tomographic means.