Thursday, February 17, 2011

Hot Rock

Kepler-10b is the name of the first confirmed rocky planet that was discovered by NASA’s Kepler space telescope. The star around which Kepler-10b orbits is an old Sun-like star designated Kepler-10. This star has an estimated age of 12 billion years and it is located at a distance of 560 light years away. Kepler-10b orbits its host star at a distance of only 3.45 stellar radii, taking just 20 hours to complete one orbit! This means that Kepler-10b transits in front of its host star once every 20 hours. During each transit event which lasts for a duration of 1.81 hours, Kepler-10b induces a 152 parts-per-million dimming of its host star. Being so near to its host star, Kepler-10b is certainly tidally locked where the same hemisphere of the planet is perpetually locked to face its parent star. Therefore, one hemisphere of Kepler-10b is in perpetual daylight while the other hemisphere experiences eternal night. The equilibrium temperature on the dayside hemisphere of Kepler-10b is estimated to be over 1833 degrees Kelvin, which makes it hot enough to melt iron.

The diameter of Kepler-10b is measured to be 1.4 times the diameter of the Earth and the mass of Kepler-10b is estimated from radial velocity measurements to be 4.6 times the mass of the Earth. This gives Kepler-10b an estimated mean volumetric density of 8.8 metric tons per cubic meter, making it on average 1.6 times denser that the Earth. Within two standard deviations of its derived mass and diameter, Kepler-10b is unequivocally a high density rocky planet with a large fraction of its mass being in the form of iron. The official paper announcing the discovery of Kepler-10b is entitled “Kepler's First Rocky Planet: Kepler-10b” and it can be obtained from

As Kepler-10b orbits its host star, the observed amount of reflected starlight from the planet will vary because the planet will present different proportions of its illuminated hemisphere during different phases in its orbit. The amount of reflected starlight from the planet will be the lowest when the planet is directly in front of the star because a distant observer will be looking entirely at just the night side of the planet and none of the day side of the planet will be visible. On the contrary, the amount of reflected starlight from the planet will be the highest when the planet is almost directly behind the star because almost all of the illuminated day side of the planet will be visible. However, the amount of reflected starlight from the planet will be zero when the planet is directly behind the star as the planet will be blocked by the star.

NASA’s Kepler space telescope observed a 7.6 parts-per-million phase curve amplitude for the total photometric output centred on the host star of Kepler-10b each time the planet makes one orbit around its host star. Furthermore, as Kepler-10b passes directly behind its host star, a 5.8 parts-per-million dip in the total photometric output is observed. This gives Kepler-10b an estimated effective geometric albedo of 0.61 which makes the planet unusually reflective because the only objects in our Solar System with such a high albedo are the planet Venus with its reflective layer of photochemically induced hazes and Saturn’s moon Enceladeus with its global coat of fresh ice. One explanation for the high albedo of Kepler-10b is a reflective layer of silicate clouds that cover the entire day side of the planet. Because Kepler-10b is such a hot world, the silicate clouds are basically clouds that are comprised of tiny suspended droplets of molten rock.