Sunday, January 19, 2014

Kepler-51 is a fairly young star with an estimated age of
~300 million years and it is also slightly more luminous than the Sun.
Observations of Kepler-51 by NASA’s Kepler space telescope found that it hosts three
transiting planet candidates - Kepler-51 b, Kepler-51 c and KOI-620.02. The
three planets have orbital periods of 45.2 days (Kepler-51 b), 85.3 days
(Kepler-51 c) and 130.2 days (KOI-620.02), placing them close to a 1:2:3
resonance. By measuring the amount of light each planet blocks as it transits
its host star, the size of each planet is found to be 7.1 (Kepler-51 b), 9.0
(Kepler-51 c) and 9.7 (KOI-620.02) times the Earth’s diameter.

As the three planets circle their host star, they
gravitationally perturb one another. This leads to transit timing variations
(TTVs) where each planet transits the host star at slightly earlier or later
timings, deviating somewhat from strictly periodic transit intervals. By
studying the TTVs, Masuda (2014) derived the mass for each of the three planets
to be 2.1 (Kepler-51 b), 4.0 (Kepler-51 c) and 7.6 (KOI-620.02) times the
Earth’s mass. With the size and mass of each planet known, all three planets were
found to have remarkably low densities of less than 5 percent the density of
water, possibly the lowest densities yet determined for exoplanets. In
comparison, the Earth has a mean bulk density of 5.52 times the density of
water. With this finding, the Kepler-51 system serves as yet another example of
a very low-density compact multi-transiting planetary system.

The planets around Kepler-51 have mean densities that are
much lower than any of the planets in the solar system. To explain their
“puffiness”, each planet probably possesses an extended outer hydrogen-helium
envelop surrounding a denser core. Assuming the planetary system has an age of
~300 million years; calculations show that the observed radii of the Kepler-51
planets can be explained if they have about 10 percent (Kepler-51b), 30 percent
(Kepler-51c) and 40 percent (KOI-620.02) of their masses in their
hydrogen-helium envelopes. All three planets are unlikely to be habitable, at
least for the type of life found on Earth, given that the planets have thick
gaseous envelopes and equilibrium temperatures that exceed 100°C.

Reference:

Masuda (2014), “Very Low-Density Planets around Kepler-51
Revealed with Transit Timing Variations and an Anomaly Similar to a
Planet-Planet Eclipse Event”, arXiv:1401.2885 [astro-ph.EP]

Saturday, January 18, 2014

M. Gillon et al. (2014) report the discovery of WASP-103 b,
an ultra-short-period planet at the edge of tidal disruption. WASP-103 b orbits
an F-type star at a distance of just ~2 stellar radii from the star's surface,
taking a mere 22.2 hours to complete an orbit. The WASP transit survey is
sensitive to detecting ultra-short-period giant planets when these planets
happen to cross in front of their host stars. WASP-103 b has 1.49 times the
mass and 1.53 times the diameter of Jupiter. This newfound planet joins a small
group of gas giants that are known to be at the verge of being tidally
disrupted by their host stars. The group include planets such as WASP-12 b and
WASP-19 b.

Artist’s impression of a gas giant. Credit: Daniel Mallia.

WASP-103 b is significantly inflated and has a bulk density
that is only 55 percent the density of water. The low density of WASP-103 b is
not just because of the intense irradiation it receives due to its extreme
closeness to its host star. Tidal heating is also expected to contribute
significantly to the planet's "bloatedness" since the planet's orbit
is only 15 to 20 percent away the Roche Limit. Any closer, the planet is
expected to be tidally destructed by the gravity of its host star.

Ultra-short-period gas giants that are right at the edge of
being tidally disrupted might experience mass loss and significant tidal
distortion. One such planet, WASP-12 b, is known to be surrounded by planetary
material that has escaped it. In the case of WASP-103 b, the extreme irradiation
it receives, the planet's inflated size and the brightness of its host star
makes it favourable for atmospheric characterisation with existing ground-based
and space-based telescopes. Observing signs of mass loss and tidal distortion
for such extreme planets can shed light on the final stages in the lives of
hot-Jupiters.

Thursday, January 9, 2014

Brown dwarfs are sub-stellar objects that are not massive
enough to fuse hydrogen in their interiors and shine as full-fledged stars.
Nevertheless, brown dwarfs are thought to form in the same way as stars do -
from collapsing clouds of gas and dust. A study by Lee et al. (2013) of an
isolated dense molecular cloud core, L328, shows that it contains three
sub-cores. One of which, identified as L328-IRS, is a Very Low Luminosity
Object (VeLLO) that is believed to be in the process of collapsing to form a brown
dwarf.

Artist’s impression of a young brown dwarf that is in the
process of accreting matter. A pair of bipolar jets can be seen stemming from
it. Credit: ESO.

Observations of carbon monoxide as a tracer for the motion
of matter reveal a bipolar outflow stemming from L328-IRS. By analysing the
outflow, the accretion rate of the proto-brown dwarf is found to be an order of
magnitude less than the accretion rate for standard star formation, consistant
with the formation of a brown dwarf. Based on the accretion rate, L328-IRS is
expected to grow to no more than ~0.05 solar mass. However, the accretion rate
may be uncertain due to several unknown factors of the outflow itself.

Nonetheless, L328-IRS has a small total envelop mass of
~0.09 solar mass and ~100 percent star formation efficiency is also unlikely.
As a result, L328-IRS is expected to be a proto-brown dwarf since it is
unlikely to accrete more than ~0.08 solar mass, which is the minimum mass
necessary to become a full-fledged star. The three sub-cores in L328 are though
to have formed concurrently in a gravitational fragmentation process. In one of
the sub-cores, global contraction of the gaseous envelop is underway to form
the proto-brown dwarf L328-IRS. All these indicate that the formation of L328-IRS
is consistant with the idea that brown dwarfs form like normal stars.

Tuesday, January 7, 2014

Parviainen H. et al. (2014) report the discovery of a massive
high-density planet on a close-in 3.58 day orbit around a 4.2 billion year old
Sun-like star. The planet is identified as CoRoT-27b. Like Jupiter, CoRoT-27b
is a gas-giant planet. Its presence was detected by the CoRoT space telescope
as the planet periodically transits its parent star and blocks a small fraction
of the star’s light. CoRoT-27b weighs in at 10.39 ± 0.55 Jupiter-masses and has
1.01 ± 0.04 times the radius of Jupiter. This gives CoRoT-27b a mean density of
12.6 times the density of water, which is more than twice the mean density of
Earth and almost 10 times the mean density of Jupiter.

Figure 1: Artist’s impression of a gas-giant planet.

Like Jupiter, CoRoT-27b is a gaseous planet comprised
primarily of hydrogen and helium. The structure and composition of CoRoT-27b
can be inferred from two models. For the first model, the planet is assumed to
be made of a central rocky core surrounded by an extensive hydrogen-helium
envelop. The 1st model is consistant with a heavy element mass fraction of 0.11,
representing a core mass of 366 Earth-masses. For the second model, a central
rocky core is absent and the heavy elements are present throughout the hydrogen-helium
envelop. The 2nd model is consistant with a heavy element mass fraction of 0.07,
representing a heavy element mass of 219 Earth-masses.

CoRoT-27b falls within a sparsely populated overlapping mass
regime between the most massive planets and brown dwarfs. Given its high mass,
gravity on the “surface” of CoRoT-27b is 27 times the surface gravity on Earth.
Technically, CoRoT-27b does not have a surface since it is gaseous through,
right down to a central rocky core, if one is present. Being so near to its
parent star, the equilibrium temperature on CoRoT-27b is estimated to be 1500 ±
130 K. The discovery of CoRoT-27b is an important addition to a scarcely populated
class of massive close-in planets.

Figure 3: Transit light curve showing the amount of dimming of
the parent star when CoRoT-27b passes in front of it. This information allows
the size of the planet to be measured. Parviainen H. et al. (2014).

Figure 4: CoRoT-27b mass, period and density compared with
the population of confirmed transiting exoplanets. Parviainen H. et al. (2014).