Low Density Planets of Kepler-51

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.

Figure 1: Artist’s impression of a gaseous planet.

Figure 2: Phase-folded transit light curves of Kepler-51 b (top), Kepler-51 c (middle) and KOI-620.02 (bottom). Black dots are the observed fluxes and coloured solid lines show the best-fit models.

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]

At the Edge of Destruction

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.

Reference:

M. Gillon et al. (2014), "WASP-103b: a new planet at the edge of tidal disruption", arXiv:1401.2784 [astro-ph.EP]

Birth of a Brown Dwarf

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.

Reference:

Chang Won Lee et al., “Early Star-forming Processes in Dense Molecular Cloud L328; Identification of L328-IRS as a Proto-brown Dwarf”, ApJ, 777:50 (15pp), 2013 November 1

CoRoT-27b: A Massive and Dense Planet

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 2: Radial velocity curve showing how much CoRoT-27b gravitationally tugs at its parent star. This information allows the planet’s mass to be estimated. Parviainen H. et al. (2014).

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).

Reference:

Parviainen H. et al. (2014), “Transiting exoplanets from the CoRoT space mission XXVII. CoRoT-27b: a massive and dense planet on a short-period orbit”, arXiv:1401.1122 [astro-ph.EP]