New exoplanet discoveries continue to make headlines. Recent studies identified the presence of water in the atmospheres of five hot-Jupiter planets. Data from Kepler revealed a hot-Earth planet that should not exist given its size and orbital location. Direct observations found a planet orbiting a different star, but again its orbital location makes its existence hard to explain. These discoveries highlight the tremendous advances in exoplanet hunting as well as the diversity of planet configurations.
The Christmas season usually brings the arrival of presents. Although I have few real needs, I enjoy opening these gifts. Mainly, the anticipation of what the gift might contain causes my excitement. Will I find something I asked for or will unwrapping reveal an unexpected surprise? Recent observations of exoplanets bring the same anticipation.
One anticipated find concerns the presence of water on distant planets. Using the powerful Hubble Space Telescope, a team observed the spectra of light present during transits from three different exoplanets: WASP-12b, WASP-17b and WASP-19b. These three planets are known as “hot Jupiters.” They have masses similar to Jupiter but orbit their host stars with very fast periods, on the order of one to four days. Because of the close distances related to these orbits, the heat from the star expands the planet atmosphere. Precision observations of the light from the star reveal what wavelengths are absorbed as the light passes through the planet’s atmosphere. Analysis performed on these three planets revealed the presence of water in each planet’s atmosphere—although the amount of water varies from planet to planet.1 Astronomers expected to find water on exoplanets (it is, after all, one of the most abundant molecules in the universe), but the varying amounts of water was somewhat unexpected. The variation provides a tool to better understand the conditions surrounding the formation and migration of the planets in these systems.
Two surprises also came from observations of planets that shouldn’t exist. The Kepler satellite found an Earth-sized planet (though 70 percent more massive, 20 percent larger in radius, and nearly equal to Earth’s density)2 orbiting a Sun-like star. The similarities to our solar system end there. Kepler-78b orbits its star with a period of 8.5 hours and has a dayside surface temperature well over 4,000°F! Although other “hot Earths” exist, this particular planet poses a problem for planet formation models. Not enough material existed at Kepler-78b’s orbital location to form a planet of its size and any reasonable model, where it forms elsewhere then migrates inward, ends with the planet crashing into the star.
A similar but opposite problem faces a planet detected using the direct imaging technique. In this instance, astronomers found a Jupiter-mass planet, labeled HD 106906b, orbiting a single star at a distance of 650 astronomical units3 (one astronomical unit corresponds to the distance from the Sun to Earth). This configuration also poses challenges for formation models. The core-accretion and disk-collapse planet formation models each fail to explain HD 106906b’s existence because they either occur too slowly or lack sufficient disk material at the proper distance from the star to produce a planet in such a wide orbit. Although binary star systems often form with similar separations, they rarely, if ever, produce objects with such a small mass ratio (<1 percent in this case whereas binary models have a lower limit around 10 percent).
These expected and surprising findings will help scientists better understand the wealth of processes and conditions involved in planet formation. This understanding will also help illuminate the rarity of planets like Earth, ones that host an abundant and diverse collection of life.