Try to imagine Saturn and Uranus orbiting the Sun in 8 and 12 days respectively. Far-fetched? In our solar system, yes, but something very similar has been observed in an exoplanetary star system, as was recently discussed by scientist and blogger Hugh Osborn, one of the co-authors of a study of the surprising 2-planet system.
In his blog post, Osborn notes re the March 2015 solar eclipse:
Calculating something so far ahead seems like an impressive feat but in fact astronomers can precisely work out exactly when and where eclipses will occur for not just the next hundred, but the next million years. Such is the way for most transiting exoplanets too, the calculations for which could probably be valid in thousands of years.
But a new planetary system, discovered by a team that includes Warwick astronomers (including me), doesn’t yet play by these rules. It consists of two planets orbiting their star, a late K star smaller than our sun, in periods of 7.9 and 11.9 days. The pair have radii 7- and 4-larger than Earth, putting them both between the sizes of Uranus and Saturn. They are the 4th and 5th planets to be confirmed in data from K2, the rejuvenated Kepler mission that monitors tens of thousands of stars looking for exoplanetary transits. (36 other planet candidates, including KIC201505350b & c, have been released previously).
But it is their orbits, rather than planetary characteristics, that have astronomers most excited. “The periods are almost exactly in a ratio of 1:1.5” explains Dave Armstrong, lead author of the study. This can be seen directly in how the star’s brightness changes over time. This lightcurve appears to have three dips of different depths, marked here by green, red and purple dips. ”Once every three orbits of the inner planet and two orbits of the outer planet, they transit at the same time”, causing the deep purple transits.
But this doesn’t just make for an interesting lightcurve; the closeness of these periods to a 3/2 ratio also causes other weird effects. “The planets perturb each other and change their period every orbit, so they never quite transit when you expect”, explains Armstrong. These shifts are called Transit Timing Variations (or TTVs).
The size of these TTVs is related to the mass of the planets, and some previous multi-planet systems have been weighed in this way. When the team went back to observe the larger planet less than 9 months later, they found that the transit time had shifted by more than an hour. And their period ratio of 1.5035 means the resulting TTVs are likely to continue increasing over a few years, potentially shifting the system more than a day from its current rhythm.
Full blog post including graphics and animations:
Shifting Eclipses – K2′s Second Multi-planet System | Lost in Transits.
Paper – One of the closest planet pairs to the 3:2 Mean Motion Resonance,
confirmed with K2 observations and Transit Timing Variations: EPIC201505350
Exoplanet discoveries – animation: