Using the most recent results from the Kepler space telescope, scientists from UNLV and the SETI Institute, which searches for intelligent extraterrestrial life, have identified a new kind of planetary system.
UNLV astrophysicist Jason Steffen and SETI scientist Jeffrey Coughlin have shown that there must be a population of planetary systems whose formation or dynamical history are distinct from their counterparts across the galaxy. The results of their study, “A Population of Planetary Systems Characterized by Short-period, Earth-sized Planets,” will appear in the Proceedings of the National Academy of Sciences.
The key feature of these systems is an isolated, very hot, rocky planet.
“We’ve shown that a large fraction of systems with hot earths can’t have the same makeup as other planetary systems discovered so far,” Steffen said. “They aren’t like the solar system, they aren’t like most Kepler systems, and they aren’t false positives.”
The best analogy, he indicated, is the population of hot jupiters — giant planets on three-day orbits that dominated the initial discoveries in the field two decades ago. Hot jupiter systems are widely viewed as having had a major difference in their formation and evolutionary past compared with other systems, and a variety of theories have been put forward to explain their origins. The number of hot earth systems is similar in number to the hot jupiters and may yield a similar advancement in our understanding of the processes involved in making planets.
To identify this new group of planets, Steffen and Coughlin relied on the process of elimination. Starting with a sample of about 150 hot earth systems, they systematically tallied the number that could be from known origins – eclipsing binary stars, noise in the data, “typical” planetary systems, and other sources.
“When we were done counting,” said Coughlin, “we still had about 20 percent that were left over — at least one in six of these systems has a different story to tell.”
The scientists noted a few existing theories that may explain the origins of these systems. They may be the leftover planet cores of hot jupiters, where the giant planet lost its large atmosphere to the central star. They may be the consequence of interactions between the planets and the last vestiges of the gas disk from which they formed. They may result from strong dynamical interactions from a newly formed system where the planet’s orbit eventually passes very close to the central star and is captured as its orbital energy is dissipated through tides. Or, they could come from some other process not yet considered.
While the origin of these systems is not known, more information about them should be forthcoming. NASA’s Transiting Exoplanet Survey Satellite (TESS) mission, which is slated for launch within the next few years, should find many similar systems that can be studied in more detail using ground-based instruments. (Kepler targets are often too dim for such follow-up observations). As scientists learn more about these systems, the information gathered should provide additional clues to their past, and help researchers better understand how unique our own solar system is, or isn’t.
“We are hopeful that this, and future studies, will steer us toward a more complete picture of how planets form and how the systems then evolve.” said Steffen. “Finding and understanding different planetary systems can tell us a lot about our own origins and how we fit into that picture.”