Friday, 19 April 2013

The nature of the Kepler-62 and Kepler-69 systems

The nature of the Kepler-62 and Kepler-69 systems, On Apr. 18, 2013, astronomers announced the discovery of five planets orbiting a star known as Kepler-62, as well as two planets orbiting a star known as Kepler-69. The seven planets were discovered by NASA's Kepler space telescope, which has spotted 122 exoplanets and 2,740 exoplanet candidates since it was launched in March 2009.

Kepler-62 is a star in the constellation Lyra that is 320 parsecs away from Earth. It has 0.69±0.02 times the mass of the Sun and 0.64±0.02 times the radius of the Sun. Its mean surface temperature is estimated to be 4,925±70 K, while the mean surface temperature of the Sun is 5,778 K. Kepler-62 is estimated to have an age of 7±4 billion years, compared to 4.57 billion years for the Sun. The spectral classification of Kepler-62 is K2V, which is significantly dimmer and redder than the G2V spectral classification of the Sun.

Kepler-69 is a star in the constellation Cygnus that is 830 parsecs away from Earth. It has 0.81±0.09 times the mass of the Sun and 0.93±0.18 times the radius of the Sun. Its mean surface temperature is estimated to be 5,638±168 K. The spectral classification of Kepler-69 is G4V, which is slightly dimmer and redder than the spectral classification of the Sun.

The seven planets that have been discovered have been given the names Kepler-62b, Kepler-62c, Kepler-62d, Kepler-62e, Kepler-62f, Kepler-69b, and Kepler-69c.

Kepler-62b has an estimated mass of 4 Earth masses and an estimated radius of 1.31 Earth radii, giving it a estimated density of about 9 g/cm^3, which is almost twice the density of Earth. These values give Kepler-62b a surface gravity of about 22.8 m/s^2, so a person who weighs 200 pounds on Earth would weigh 466 pounds on Kepler-62b. It is 0.0553 AU away from its star, and completes an orbit of Kepler-62 once every 5.715 Earth days. For comparison, Mercury is 0.387 AU away from the Sun on average, and orbits the Sun once every 87.969 Earth days. The black-body temperature of Kepler-62b is 707 K or 434°C, which is close to the surface temperature on Venus. Complex life as we know it would be impossible on this planet, as liquid water can only exist at temperatures below 647 K. The actual temperature of any planet will be warmer than its black-body temperature due to albedo, atmosphere, and internal heating effects.

Kepler-62c has an estimated mass of 5 Earth masses and an estimated radius of 0.54 Earth radii, giving it a estimated density of about 195 g/cm^3, which is greater than the density of any object in our solar system. These values give Kepler-62c a surface gravity of about 167.7 m/s^2, so a person who weighs 200 pounds on Earth would weigh 3,429 pounds on Kepler-62c. It is 0.0929 AU away from its star, and completes an orbit of Kepler-62 once every 12.442 Earth days. The black-body temperature of Kepler-62c is 573 K or 300°C, which is consistent with an oven set to broil. Extreme pressure from gravity and the planet's atmosphere may make liquid water possible despite the high temperature, which would boil liquid water at atmospheric pressure on Earth.

Kepler-62d has an estimated mass of 6 Earth masses and an estimated radius of 1.95 Earth radii, giving it a estimated density of about 5 g/cm^3, which is close to Earth's density of 5.515 g/cm^3. These values give Kepler-62d a surface gravity of about 15.4 m/s^2, so a person who weighs 200 pounds on Earth would weigh 316 pounds on Kepler-62d. It is 0.12 AU away from its star, and completes an orbit of Kepler-62 once every 18.164 Earth days. The black-body temperature of Kepler-62d is 481 K or 208°C, which is close to Mercury's black-body temperature of 440 K or 167°C. Extreme pressure from gravity and the planet's atmosphere may make liquid water possible despite the high temperature, which would boil liquid water at atmospheric pressure on Earth.

Kepler-62e has an estimated mass of 19 Earth masses and an estimated radius of 1.61 Earth radii, giving it a estimated density of about 24 g/cm^3, which is greater than the density of any object in our solar system. These values give Kepler-62e a surface gravity of about 71.68 m/s^2, so a person who weighs 200 pounds on Earth would weigh 1,466 pounds on Kepler-62e. It is 0.427 AU away from its star, and completes an orbit of Kepler-62 once every 122.387 Earth days. The black-body temperature of Kepler-62e is 265 K or -8°C, which is close to Earth's black-body temperature of 254 K or -19°C.

Kepler-62f has an estimated mass of 50 Earth masses and an estimated radius of 1.41 Earth radii, giving it a estimated density of about 98 g/cm^3, which is greater than the density of any object in our solar system. These values give Kepler-62f a surface gravity of about 246 m/s^2, so a person who weighs 200 pounds on Earth would weigh 5,030 pounds on Kepler-62f. It is 0.718 AU away from its star, and completes an orbit of Kepler-62 once every 267.291 Earth days. For comparison, Venus is 0.723 AU away from the Sun on average, and orbits the Sun once every 224.698 Earth days. The black-body temperature of Kepler-62e is 205 K or -68°C, which is close to Mars's black-body temperature of 210 K or -63°C.

Kepler-69b does not yet have an estimated mass. It has an estimated radius of 2.24 Earth radii. It is 0.094 AU away from its star, and completes an orbit of Kepler-69 once every 13.722 Earth days. The black-body temperature of Kepler-69b is 779 K or 506°C, which is too hot to support liquid water at any pressure.

Kepler-69c does not yet have an estimated mass. It has an estimated radius of 1.71 Earth radii. It is 0.64 AU away from its star, and completes an orbit of Kepler-69 once every 242.461 Earth days. The black-body temperature of Kepler-69b is 299 K or 26°C, which is close to room temperature.

There are several methods that can be used to detect extrasolar planets. The three planets detected around Kepler-62 and Kepler-69 were observed by the transit method. With this method, the observed visual brightness of the star drops a small amount when the planet passes between Earth and the planet's star. This method has two major disadvantages: planetary transits are only observable for planets whose orbits happen to be perfectly aligned from the astronomers' vantage point, and the method suffers from a high rate of false detections. A transit detection requires additional confirmation, typically from the radial velocity method. The main advantage of the transit method is that the size of the planet can be determined from the lightcurve. When combined with the radial-velocity method (which determines the planet's mass) one can determine the density of the planet, and hence learn something about the planet's physical structure. These two methods were combined for the Kepler-62 planets, which allowed me to calculate the surface gravity for them, based on the available data.

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