Kepler-80: Analysis of a Compact System

It’s been a week for unusual planetary systems, and I’ll cap it off with Kepler-80, a star about 1100 light years away that features five planets in extraordinarily tight orbits. Such systems are now being referred to as STIPs (Systems with Tightly-spaced Planets), a nod to our apparently imperishable drive to create acronyms. Whatever we call them, though, systems like these make us realize that our own Solar System’s configuration is but one possibility in a sea of other outcomes. Yesterday’s post on ‘warm Jupiters’ is yet another confirmation of the thought.

What we have in new work from Mariah MacDonald, Darin Ragozzine (Florida Institute of Technology) and colleagues is an analysis of transit timing variations (TTVs) of the planets around this star, all of which orbit inside 1/10 AU. Here the planets’ years are 1.0, 3.1, 4.6, 7.1 and 9.5 days, respectively, close enough that gravitational perturbations can create slight changes in transit times. Although the innermost planet has a very weak TTV signal, the other four show signals strong enough for the researchers to work out the masses of each.

Gravitational interactions that disturb a perfectly periodic sequence of transits are a valuable way of making mass estimates for planets small enough that radial velocity detections are difficult. Usefully, Kepler has measured hundreds of TTV signals allowing for such estimates. They’re particularly helpful in multiple-planet transiting systems because now we can use the combination of mass and planetary radius to produce density measurements.

The Kepler-80 planets are f, d, e, b, and c in order of period. The inferred masses for the four outer planets are roughly 6.75, 4.13, 6.93 and 6.74 Earth masses, but we learn that the two outermost planets are almost twice as large as the inner two. The researchers believe this is consistent with terrestrial compositions for d and e and extended, puffy atmospheres of hydrogen and helium for b and c. Here’s how the paper describes these worlds:

Although all four planets have very similar masses, planets d and e are terrestrial and planets b and c have ∼2% (by mass) H/He envelopes assuming Earth-like cores. Their orbits are similar and models suggest that photo-evaporation would have removed ∼1% H/He from all four planets. Though simulations suggest the system has been affected by planetary tides, we did not consider the effect of dissipation on the atmospheric history of the planets. It is unusual to have four well-measured densities in the same system and future comparative planetology may constrain the formation and evolution of their atmospheres.

Due to orbital resonances, the four outer planets are synchronized, returning to the same configuration every 27 days. The paper notes that Kepler-80’s planetary orbits are stable in the long-term as long as we assume orbital eccentricities below about 0.2 (the researchers point out that TTVs cannot reliably detect eccentricities for this system). Although the available Kepler data are not enough to reveal the evolution of the atmospheres on these planets, the researchers’ simulations show that the outer two planets could have migrated inward from original positions in the disk where accretion of hydrogen and helium would be more likely to occur.

Image: This animation shows the position of the five planets of Kepler-80 whenever the outer two planets (green and red) pass by one another, about every 27 days over the course of four years of observations by NASA’s Kepler Space Telescope. Due to the rare synchronized nature of the system, the middle two planets (blue and purple) also return to almost exactly the same location. The innermost planet (yellow) is not synchronized and hence is found at a random location every 27 days. MacDonald et al. 2016 were able to show that this pattern indicates formation by “migration,” where the orbits shrink very slightly over time. The orbits are to scale with each other, but the planets are shown 50 times larger. The outer four planets are all about 4-6 times the mass of the Earth. The inner three planets (blue, purple, and yellow) appear rocky and the outer two planets (green and red) are likely rocky with a very puffy Hydrogen/Helium atmosphere. Credit: MacDonald/Ragozzine/FIT.

Improved mass and eccentricity estimates will fall to future space-based observatories. With its complex resonances and intriguing dynamical history, Kepler-80 should be a useful laboratory for studying planet formation. The Kepler mission has given us a wealth of information about how planetary systems can be built, and it’s clear that their formation and evolution will be the subject of study for decades. The systems we’ve looked at this week hint at what is possible as exoplanetary architectures continue to surprise us.

The paper is MacDonald et al., “A Dynamical Analysis of the Kepler-80 System of Five Transiting Planets,” accepted at The Astronomical Journal. A Florida Institute of Technology news release is available.