Large planets very close to their star, known as hot Jupiters, are the easiest planets to find and make up a disproportionate share of the exoplanets we have discovered. Although that share is dropping from the days when it was almost 100%, we still have plenty to learn about them, such as the unexpected effects they have on their parent stars.
Planets, even gas giants, are so much smaller than their parent stars that we usually think of the effects running one way only. However, scientists recently discovered that many hot Jupiters have orbits that don’t align with the rotational axes of their stars. It’s surprising since not only do all the planets in our solar system orbit in a plane at nearly 90° to the Sun’s axis, but our understanding of star formation also suggests this should be the norm. The findings were published in a paper in Earth and Planetary Astrophysics.
It is considered impossible for planets, let alone giant ones, to form as close to their stars as hot Jupiters now lie. Lead author Natalia Storch, a graduate student at Cornell, modeled the migration of large planets formed at Jupiter-like distances to the star. She found that in the process, the star’s rotational axis would start to precess like a slowing top. Precession is a feature of the Earth’s axis, but not, as far as we know, that of the Sun.
“It can make the star’s spin axis change direction in a rather complex – or even a chaotic – way,” says Professor Dong Lai, Storch’s supervisor.
“Although the planet’s mass is only one-thousandth of the mass of the sun, the stars in these other solar systems are being affected by these planets and making the stars themselves act in a crazy way,” Lai adds. The Sun comprises just 4% of the solar system’s angular moment, Jupiter comprises 60%, so if its orbit were to change drastically it could have a similar effect on the Sun’s spin. The question remains though: what could cause such a change? After all, Jupiter’s orbit appears to have been remarkably stable for a very long time.
Storch and Lai based their models on wide binaries, that is star system where a companion star orbits at a great distance from the one around which the hot Jupiter travels. They say the planet “serves as a link between the two stars”, but argue “similar consideration can be applied to the formation of short-period stellar binaries”. However, some hot Jupiters — including the first discovered — orbit stars without companions, although not as many as was recently thought. A distant companion star can disrupt the orbits of large planets, which as a result affect the primary star’s axial tilt.