Planets of all sizes ranging from 0.2 to 25 times Earth’s diameter have been discovered. This is particularly interesting because the radii of planets in our solar system don’t have this range. Expanding the Nice model to explain the masses, sizes, and locations of exoplanets is still very much a work in progress. The smallest exoplanet discovered so far, Kepler 37b, has a diameter only slightly larger than that of our Moon. It is one of at least three planets orbiting the star, Kepler 37, which has a mass of 0.8 M_⊙.

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By observing transits, astronomers can also study the spectrum of a planet’s atmosphere, from which we learn about that atmosphere’s chemical composition. When the gas giant planet labeled HD 209458b passed in front of the star HD 209458 (Henry Draper catalog star number 209458) some 150 ly from Earth, hydrogen, sodium, oxygen, carbon, and carbon monoxide in the planet’s atmosphere absorbed certain wavelengths of starlight. Therefore, the star’s spectrum has extra absorption lines (introduced in Section 3-13). The Hubble Space Telescope further revealed that the outer layers of this planet are being heated so much that at least 9 × 10⁶ kg (10,000 tons) of hydrogen are evaporating into space from it every second. In 2010, a tail created by this departing gas was observed. Based on this rate of mass loss, the planet has lost at least 0.1% of its mass over its lifetime of 5 billion years. Furthermore, observations in 2010 revealed winds in the outer cloud layers of this planet reaching 7000 km/h (4300 mi/h), more than 3 times faster than any winds known to occur in our solar system. The atmospheres of other exoplanets also include methane, carbon dioxide, and water vapor.

The transit method has also revealed that some exoplanets have much more high-density material in their cores than do our giant planets. The changing intensity of light from the star HD 149026 showed that the planet passing in front of it, which has a mass 115 times that of Earth, is much smaller than other planets of that mass. Calculations reveal that about 70 Earth masses (roughly two-thirds) of that planet is rocky material in its core. For comparison, Jupiter’s terrestrial core is about 4% of its total mass.

In 2005, microlensing was used to detect a terrestrial (rocky) planet labeled Gliese 876d orbiting star Gliese 876. (The Gliese catalogs list nearby stars and their known planets.) This planet has 6.8 times Earth’s mass and, because it is less than twice Earth’s radius, it is classified as a super-Earth. Figure 5-7a summarizes the present naming scheme for exoplanets based on their sizes. Figure 5-7b shows the distribution of known exoplanet masses.

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