Astronomers are beginning to understand the unlikely formation and dangerous survival of exoplanets circling binary stars.
Reality is catching up with science fiction. In 2011 astronomers detected a planet orbiting two stars and nicknamed it Tatooine after the fictional Star Wars planet. To date, six similar planets have joined the list of wacky circumbinary planets.
But even with a half dozen of these systems to study, astronomers are baffled. Binary star systems are downright dangerous. Powerful tidal forces from the two stars can easily grind a planet to dust, let alone prevent it from forming in the first place.
This week, however, two binary star systems are shedding twice the light on their circling exoplanets, providing promising clues to these exotic systems.
First case study: Kepler-34b
Research led by University of Bristol astronomers show that most circumbinary planets likely formed far away from their central stars and then migrated in at some point in their history.
Take as an example Kepler-34b. At a whopping distance of 4,900 light-years, this gaseous Saturn-like planet orbits its two Sun-like stars at 1.09 astronomical units every 289 days. The stars themselves orbit and eclipse each other every 28 days.
In order to better understand how this system formed and evolved, the team carried out two computer simulations representative of the Kepler-34 stellar system and a third of a single star system.
They began with a circumbinary protoplanetary disk: a dusty circling plane of small objects, or planetesimals, where planets form. They then watched the disk’s particles gravitationally interact with one another over time (in this case 150 years, or 2,000 binary orbits) in order to determine the likelihood that planets might form.
In all three simulations the authors included gravity between the planetesimals. Both simulations of the Kepler-34 system were identical except the first contained much larger planetesimals than the second. More massive planetesimals should be harder to disrupt.
The authors looked at three types of collisions, or “events.” Growth-enabling events occur when two planetesimals collide and create one larger body. Partial and catastrophic erosion events occur when two planetesimals collide and grind each other down severely, even to dust. And hit-and-run events occur when two planetesimals collide and simply bounce off one another. Study coauthor Zoë Leinhardt (University of Bristol, UK) compares this event to a billiard ball hitting another billiard ball. There’s minimal harm in that no mass is lost or exchanged, but there’s also no growth.
Around the single star, growth-enabling events account for 48% of the collisions, partial and catastrophic erosion events for 14% of the collisions, and hit-and-run events for 38% of the collisions.
In contrast, the collision outcomes in both simulated Kepler-34 systems vary dramatically by distance from the host stars. Super-catastrophic collisions dominate out to 1.1 a.u., leading to a hostile environment. But then partial and perfect mergers start taking over. And by 1.5 a.u., the collision occurrence rates begin converging toward that of a single star.
Given the harrowing formation environment that would have existed at Kepler-34b’s current location, the planet likely formed beyond 1.5 a.u. and migrated inward. When the authors evaluated all known circumbinary planets, they found that only one planet, Kepler-47c, could have formed at its current location of 0.99 a.u.
While the research team was the first to include the self-gravity of the planetesimals and an accurate collision model, the next phase will be to include the gravitational influence of the gas in the protoplanetary disk (as opposed to only the solid planetesimals), says exoplanet researcher Hannah Jang-Condell (University of Wyoming). Leinhardt agrees this will be the next step.
Second case study: Kepler-413b
A second circumbinary planet recently in the news is located a little closer to home at 2,300 light-years. Kepler-413b orbits an orange star and a red dwarf every 66 days.
But that doesn’t mean this super-Neptune transits every 66 days like clockwork (most exoplanets we’ve detected do in fact transit consistently). Astronomers using data from NASA’s Kepler mission found an unusual pattern in the transits: they saw three transits in the first 180 days, no transits throughout the next 800 days, and then five transits in a row. No visible transits should occur again for the next six years.
The main reason for this odd pattern is that the planet does not orbit the two stars in the same plane as the two stars travel in around each other. This misalignment leads to an orbital precession — the planet’s point of closest approach to the stars slowly rotates around the system. This causes the orbit to oscillate well above and below the plane the two stars travel in around each other.
Because of this orbital precession, the incident stellar radiation on Kepler-413b varies from 0.9 times that of the Sun on the Earth to 2.7 times greater. When the exoplanet is closest to the binary star system it may be close to the primary star leading to a ton of incoming stellar radiation, or it may be close to the secondary star leading to little incoming stellar radiation, explains study coauthor Veselin Kostov (Johns Hopkins University and Space Telescope Science Institute).
This exoplanet will experience erratic changes in seasons due to drastic fluctuations in temperate from its host stars based on complex orbital dynamics.
Astronomers are still trying to explain why this planet is out of alignment with its stars. In our solar system, other planets exert gravitational influences on a single planet and cause it to precess. There could be other planets in this system, or the likely culprit could be a very powerful outside star.
S. Lines et al. “Forming Circumbinary Planets: N-Body Simulations of Kepler-34.” Astrophysical Journal Letters, 2014
V.B. Kostov et al. “Kepler-413b: a slightly misaligned, Neptune-size transiting circumbinary planet.” Astrophysical Journal, 2014