The Kepler telescope spent years watching thousands of stars, spotting planets by the tiny dips in starlight when a planet passes in front of its star. Such a passage, or transit, lets us measure the planet's size but not the shape of its path. However, if a planet follows an elongated orbit, it speeds up and slows down, and the time between light dips wobbles slightly. Those timing variations are what gave away the secret of the orbits.
A team led by Gilbert took data on 1626 planets and used Bayesian statistics (a method that carefully separates signal from noise) to estimate how stretched their orbits are. They found that small planets with periods shorter than four days have nearly perfect circular orbits — tidal forces from the star have smoothed them out, much like water polishes pebbles. This was already known for giant planets, but for super-Earths and sub-Neptunes, it's a first.
Now for the interesting part. Planets with only one visible transit (so-called 'singles') have more elongated orbits than those in systems with two or more visible planets. And two-planet systems have more elongated orbits than those with three or more. Comparing these data with computer models of planet formation, the astronomers concluded that 'singles' actually have, on average, about three neighbors: we just don't see them because of orbital inclination. 'Pairs' have even more neighbors: between four and six.
Interestingly, no correlation was found between orbital shape and the distances between planets, the spread of their sizes, or their ordering. This suggests that many planetary families grew up quietly, without violent collisions or 'gravitational brawls' that could stretch orbits. In the future, with more refined analysis methods, we might even catch faint traces of a turbulent youth.
Mercury, the closest planet to the Sun, has the most elongated orbit of all planets in our solar system: its distance from the Sun ranges from 46 to 70 million kilometers. Earth's orbit, by contrast, is almost circular, the difference is just 5 million kilometers.
In Liu Cixin's novel 'The Three-Body Problem,' a planet with a chaotic orbit around three stars endures eras of unbearable heat and freezing cold, a vivid example of how orbital shape drives climate.