Солнечная система и ее тайны

Планеты Созвездия НЛО


Many of the exoplanets discovered in the past few years have been measured with both the transit method and the wobble method. With size (from the transit) and mass (from wobble), we can apply the simple formula for the volume of a sphere, and determine the mean density of the planet, ?planet:

This is a powerful way to infer a planet's composition from a distance of hundreds of light years away. When the NASA Kepler mission team discovered transits of two different small planets orbiting a distant star, we designated as Kepler-10, the mass of the inner planet (Kepler-10b) was determined to be about 4.5 Earth masses. Its transits revealed a planet radius of only 1.4 Earth radii, therefore its mean density was 8.8 grams/cm3.

This is very dense! Consider this: the density of rocks on Earth's surface can hardly reach 3.0 grams/cm3, and pure iron is "just" about 7.9 grams/cm3; you'd need to consider copper (at 8.9 grams/cm3) to match the mean density of Kepler-10b. What is going on? Do we conclude that Kepler-10b is a bulk copper planet?

The answer is no. The mean densities above are valid for low pressure environments, such as on the surface of a planet. The interiors of planets are compressed to monstrously high pressures, which causes all materials to become denser. For example, the mean density of Earth is 5.5 grams/cm3 with a composition of roughly 2/3 rocks and 1/3 metal iron. However, Kepler-10b has 4.5 times more mass packed into only a 40% larger planet. This makes the central pressure more than 10 times higher than inside the Earth, producing a much higher mean density from the same overall composition. In conclusion, Kepler-10b is a rocky planet with composition similar to Earth (see figure below, left panel).

In November 2013 the Kepler team found a planet that resembles Earth even more closely in terms of its internal composition, Kepler-78b. Both Kepler-10b and Kepler-78b are unusual planets, orbiting their stars extremely closely thus being incredibly hot on their surface.

Some of the planets discovered recently have mean densities lower than that of rock, but much higher than that of hydrogen and helium gas (the bulk composition of giant gas planets like Jupiter and Saturn). They are a new type of planet to us, unknown in the Solar System, and we call them Water planets or Water worlds (see figure above, right panel). We do not have direct evidence that the low density material is actually water (H2O), but this is a very safe assumption. As long as a planet forms far enough from the star to accumulate lots of water, and grows massive enough to retain its water, it is not surprising to find a true water world.

It is also natural to assume that the interiors of small planets (similar to Earth, Venus, and Mars) have the onion-like layered structure seen in the figure above. This structure develops during the birth of the planet when it is still mostly molten, and heavier materials that are not chemically bound to lighter minerals would sink deeper. When we determine the mean density of an exoplanet, we use computer models of the layered structure to infer the fractions of different materials required to match the observations. We can then determine whether the exoplanet is a rocky one or a water world, or perhaps even a gas enshrouded planet.

Солнечная система и ее тайны