Planet formation occurs readily throughout the galaxy—but that does not mean that solar systems like ours are equally abundant. The last 20 years of exoplanet studies highlight the numerous ways that the formation of planets around stars, other than the Sun, results in planetary systems incapable of hosting life. Astronomers also recently discovered a Jupiter-like planet floating through space. Closer study of this object demonstrates that scientists can distinguish high-mass planets from low-mass stars and that processes can eject planets from the pull of the host star, likely disrupting the rest of the planetary system as well.
Middle school is often a difficult time for students. I remember being one of five boys in a classroom with tables that sat four. Sitting at the table with three other kids, I noticed the fifth boy sitting at a table all by himself. As I moved over to his table so that everyone would have company, he seized the opportunity and moved into the seat I had vacated. Despite my desire to include everyone, I ended up alone. Planets also face difficult challenges due to interactions with other planets and stars—and the latest research indicates they often end up alone, floating through space.
Using a survey primarily sensitive to young, small stars, astronomers recently found an object dubbed PSO J318.5-22. Detailed analysis of this object led the researchers to conclude that it has a mass 6.5 times the mass of Jupiter and a surface temperature of 1160K (1628oF). Coupled with the spectral features of the object’s light (most notably, the lack of methane signatures) and its likely membership in the β Pic moving group of stars, PSO has an age near 12 million years.1
While PSO J318.5-22 moves through space like a star, it exhibits all the characteristics of a planet. In fact, when astronomers compare the light signatures from PSO J318.5-22 with the light from planets in the HR 8799 system, they find great similarity. This result implies two things. First, PSO J318.5-22 appears to be a planet, not a star. Second, since planets form around stars, some process dislodged PSO J318.5-22 from its host star.
Because PSO now wanders around space unattached to a star, astronomers have the opportunity to study its light in much more detail than normally possible. For planets like those around HR 8799, the light from the star overpowers the light from the planet, making such observations difficult. Without this obstacle to overcome, telescopes can collect detailed information about the planet that will help determine how it formed and understand its future development.
The direct observation of a planet freely floating through space also buttresses the idea that planets in young systems encounter circumstances that drastically alter the planetary system configuration. For example, some scientists argue that our solar system started with five gas giants (Jupiter, Saturn, Neptune, Uranus, and one more). However, dynamics in the early solar system ejected the fifth planet, changed the orbits of the other four, and cleaned out the comet and asteroid debris leftover from the formation of the system.
Given that these dynamic changes resulted in changes to the planetary system, it’s reasonable to infer that the changes usually bring damage and destruction. Yet in the solar system, they brought benefits, namely the increased the capacity of Earth to support life—almost like they happened by design.
1. Michael C. Liu et al., “The Extremely Red, Young L Dwarf PSO J318.5338-22.8603: A Free-Floating Planetary-Mass Analog to Directly Imaged Young Gas-Giant Planets,” Astrophysical Journal Letters, in press, arXiv:1310.0457.