How long can the most extreme planetary systems survive? Measuring the tidal orbital decay of hot Jupiters.

For centuries, humanity has dreamed of planets beyond the Earth, and even beyond our Solar System. In 1995, these dreams started to become a reality, with the discovery of the first planet orbiting another star like our Sun. This exoplanet was remarkable because it looked very different to anything we were used to. The planet is a massive gas giant, similar to Jupiter, but takes just 4 days to complete an orbit of its star (contrast this with 88 days for Mercury, the closest planet to the Sun, and almost 12 years for Jupiter). Since then, many more such planets have been discovered, some even closer to their star (the record holder has an orbital period of just 18 hours). These exoplanets are known as ‘hot Jupiters’.

One of the biggest questions we have about these short-period hot Jupiters is how long they can survive so close to their host star (50 or more times closer than Earth is to the Sun). At these distances there are strong tidal forces at work between the planet and the star (rather like those in action between the Earth and the Moon). These forces cause the planet to lose energy which is transferred to the star. This loss of energy from the planetary orbit causes the planet’s orbit to get gradually smaller, and the planet slowly spiral towards the star until it is destroyed. What we don’t know is how fast this process of tidal orbital decay is. It might be that this happens relatively rapidly, with hot Jupiters only living for a few million or tens of millions of years, which is less than one percent of the star’s lifetime. Alternatively, this slow dance of death might take billions of years, and so the life expectancy of a hot Jupiter would be similar to that of its star. This timescale is determined by a quantity Q*, which tells us how efficient a star is at absorbing the energy from a planet’s orbit. Unfortunately, we don’t know how big Q* is; in fact estimates vary hugely, which is what causes the enormous uncertainty in hot Jupiter lifetimes.

This project aims to accurately measure Q* for several planetary systems. We will use exoplanets that pass directly in front of their host stars (transit) as viewed from Earth. Normally, we expect to observe these transits spaced at exactly regular intervals as the planet transits once per orbit. If the orbit is undergoing tidal decay, however, we expect to see the transits getting ever-so-slightly closer together. The effect is predicted to be just a few seconds shift, measured over a period of several years. This is, however, possible with modern telescopes, cameras and advanced data reduction and modelling techniques.

Measuring Q* will not only tell us the life expectancy of hot Jupiters, it will also tell us about the timescales of other processes resulting from tidal interactions between star and planet. This will, for example, help us to determine how the hot Jupiters came to exist so close to their host stars when we think they must have formed further away, where it is cooler.