Older planets, more eccentric?

by Sam Grunblatt

Many people get more eccentric as they enter old age. Why should planets be any different?

For the past 4 years, the K2 extension to the NASA Kepler mission has been hunting for planets on small orbits around a wide range of stars, including stars much hotter, much cooler, and almost exactly the same as the Sun. One of the stellar populations studied were red giant stars, which our Sun and other stars like it will evolve into in about 4 to 6 billion years. 

My team and I were discovered two gas giant planets around these evolved stars using data from K2. We then got additional radial velocity measurements from the ground in order to measure the mass and better understand the orbits of these planets. We found that both of these planets were on eccentric, non-circular orbits, a result we weren’t expecting. 

This inspired us to investigate the population of all known giant planets orbiting evolved stars and compare it to the same population orbiting un-evolved, Sun-like stars. We found that on average, giant planets with periods less than 50 days are significantly more likely to be eccentric around evolved stars than around un-evolved stars. 

We’re not totally sure why this is the case, but it likely has something to do with tides on both the star and the planet changing significantly as the star’s radius increases. And with the recent launch of NASA’s new TESS mission, which will observe the entire sky for similar systems over the next 2 years, we will likely find tens to hundreds more of these evolved systems. This will create a large enough sample to determine how long it took these planets to become eccentric, and what kinds of original orbits the planets came from. Soon, we will understand what makes these aging giant planets so eccentric!

Planet eccentricity versus orbital period. Evolved, or giant star systems are shown in red, while un-evolved, or dwarf stars systems are in black. Point size is proportional to stellar radius. The three red stars are three systems our team obtained radial velocity measurements for. The red and black lines show the median eccentricities for evolved and un-evolved stars at each period. At small orbital periods, the median eccentricities of both populations do not overlap.