5-6 Many exoplanets have extraordinary orbits, as compared to those in our solar system

In 2003, astronomers used the transit method to detect a planet only 0.024 AU from its star (one-sixteenth as far from its star as Mercury is from the Sun). That planet orbits once every 28 hours and 33 minutes. In 2011, the planet Kepler 70b was observed orbiting its star once every 5.8 hours at a distance of 0.01 AU. This planet has a surface temperature of 7200 K, hotter than the surface of our Sun. It is the planet closest to its star known so far.

In 2010, observations of exoplanets transiting their stars revealed that some of the planets are orbiting in the opposite direction to which their stars rotate (or spin). This motion is intriguing because it runs counter to the predictions of the Nice model (see Section 4-2), in which planets should orbit in the same direction as the stellar rotation, as occurs here in our solar system.

Many of the more than 1800 exoplanets discovered so far have highly elliptical orbits, with eccentricities up to at least e = 0.97, compared to the highest orbital eccentricity of any planet in our solar system of 0.206 for Mercury. For example, Fomalhaut b, discussed in Section 5-2, is a planet with a high orbital eccentricity. Star systems with massive planets in highly eccentric orbits are unlikely to have life-sustaining Earthlike planets, because the changing location of the massive planet is likely to prevent smaller planets from staying in stable orbits.

As you can see in Figure 5-8a, many giant planets orbit surprisingly close to their stars, much closer than Earth is to the Sun. If the Nice model of how giant planets form is correct, then these planets are spiraling inward after having formed farther away from their stars than they are now.

A dramatic consequence of the inward spiraling of planets was discovered in 2001, when the remnants of at least one planet were discovered in the atmosphere of star HD 82943 that still has at least two other planets in orbit around it. This star’s atmosphere contained a rare form of lithium that is found in planets but that is destroyed in stars within 30 million years after they form. The presence of this isotope, 6Li, means that at least one planet spiraled so close to the star that it was vaporized.

In 2008, a roughly Jupiter-mass planet, WASP-12b, was discovered so close to its star, WASP-12, that tides caused by the star have made the planet egg-shaped. WASP-12b’s atmosphere is now being pulled onto its star at a rate of about 15 × 1020 kg (2 × 1017 tons) per year. The planet will be devoured within 10 million years.

Conversely, the exoplanet farthest from its star is HIP 77900 b, which orbits its star at a distance of 3200 AU. You can explore many facets of exoplanetology at exoplanet.eu.