The Earth’s axis of rotation is tilted 23.4 degrees to its orbital motion around the Sun (more precisely, its spin axis has a tilt of 23.4 degrees with respect to the axis of its orbit). This tilt, which causes our seasonal variations, was likely the result of a cataclysmic impact that occurred about 4.5 billion years ago between the Earth and another large body which probably also resulted in the formation of the moon. Such a large tilt is thought in general to result from a strong interaction between objects like the collision that formed the moon. Stars also spin, and their spin axes can also be tilted with respect to the orbits of their planets. In the case of our Sun, which spins roughly once every twenty-five days, its tilt is only 7.25 degrees, and so we never get a very good look at its north or south poles. Astronomers infer therefore that the Sun never had a traumatic encounter with another star (at least not since its planetary system formed, and at least not with a sudden collision).
The tilt of a star’s spin is obviously an important feature, but knowing it first requires knowing the orbit of its planets. One amazing and serendipitous result of the Kepler mission and its powerful ability to find transiting planets is its ability – by characterizing planetary systems – to address the issue of stellar tilts. Stellar tilts measured so far in exoplanetary systems display a surprising diversity, from very small values like our Sun’s to strongly tilted stars, and even some “retrograde” stars whose direction of rotation is opposite to the planetary orbital revolution. These stars also happen to be somewhat young; indeed most of the stars hosting planets in the Kepler program have not evolved past the hydrogen burning stage of their lives. Almost all of the tilted stars host hot Jupiters—planets about the mass of Jupiter (that is, large planets) but that orbit very close to the star and so are hot. The wide range of observed tilts suggests that they arose not from a standard sudden collision, but rather from some more intricate process that transports a normal Jupiter from its orbit far out (as in our solar system) to an orbit very close to the star.