On February 20, 2013, astronomers announced the discovery of three planets orbiting a star known as Kepler-37. The star is 66 parsecs away from Earth, and is the first star that has been found to have a planet whose size is on par with Earth’s moon. This contrasts with the “hot Jupiter” and “hot Neptune” planets that have been observed much more commonly so far. The three planets were discovered in September by NASA’s Kepler space telescope, which has spotted 114 exoplanets and 2,740 exoplanet candidates since it was launched in March 2009.
Kepler-37 is a star in the constellation Lyra that has 0.803±0.068 times the mass of the Sun and 0.770±0.026 times the radius of the Sun. Its mean surface temperature is estimated to be 5,417±75 K, while the mean surface temperature of the Sun is 5,778 K. The stellar rotation velocity of Kepler-37 is estimated at 1.1±1.1 kilometers per second, while the Sun rotates at 2.0 kilometers per second. Kepler-37 is estimated to have an age of 6 billion years, compared to 4.57 billion years for the Sun. The spectral classification of Kepler-37 is G8V, which is slightly dimmer and redder than the G2V spectral classification of the Sun.
The three planets that have been discovered have been given the names Kepler-37b, Kepler-37c, and Kepler-37d.
Kepler-37b has an estimated mass of 0.01 Earth masses and an estimated radius of 0.303 Earth radii, giving it a estimated density of about 2.0 g/cm^3, which is greater than all of the gas giants but less than all of the terrestrial planets in our solar system. These values give Kepler-37b a surface gravity of about 1.1 m/s^2, so a person who weighs 200 pounds on Earth would weigh 22 pounds on Kepler-37b. It is 0.1003 AU away from its star, and completes an orbit of Kepler-37 once every 13.367 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-37b is 700 K or 427°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-37c has an estimated radius of 0.742 Earth radii. It is 0.1368 AU away from its star, and completes an orbit of Kepler-37 once every 21.302 Earth days. The black-body temperature of Kepler-37c is 560 K or 287°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-37d has an estimated radius of 1.99 Earth radii. It is 0.2076 AU away from its star, and completes an orbit of Kepler-37 once every 39.792 Earth days. The black-body temperature of Kepler-37d is 460 K or 187°C, which is consistent with moderate heat inside an oven. 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.
There are several methods that can be used to detect extrasolar planets. The three planets detected around Kepler-37 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-37 planets, which allowed me to calculate the density and surface gravity for Kepler-37b, based on the available data. As of this writing, the masses of Kepler-37c and Kepler-37d are not yet available.
The discovery of the Kepler-37 system was notable for being the smallest to date to be measured with asteroseismology. In this process, astronomers probe the internal structure of a star by examining sound waves generated by convection inside the star in much the same way that geologists use seismic waves generated by earthquakes to probe the interior structure of Earth. The sound waves cause oscillations that Kepler observes as a rapid fluctuation of the star’s luminous intensity. The smaller the star, the greater the frequency of the oscillations and therefore the more rapid the fluctuation of the star’s luminous intensity. The measurement of a star that is only 80 percent as large and 77 percent as massive as the Sun with asteroseismology represents a new benchmark for success with this method.