# Fascinating orbits

Adrian Hamers is researching the way in which celestial bodies orbit each other, now and in the future. This often turns out to be more erratic than you might think. He will defend his PhD dissertation on 21 June.

For the past few years, Hamers has been looking at hierarchical systems, groups of three or more celestial bodies that orbit each other. All these groups behave according to the same laws of physics, whether they consist of supermassive black holes, planetesimals or planets.

### From circle to oval

Using mathematical calculations, Hamers can predict how the orbits of the celestial bodies will change over the next billions of years: ‘Interestingly, you can see that it’s particularly the orientation and shape of the orbits that change significantly over time, especially from circular to oval orbits.’

### High and low tide

These changes have their consequences. When the orbits become more oval in shape, the bodies will come in increasingly close proximity when passing at the shortest point. This will result in strong gravitational interactions. Hamers: ‘Gravity can cause strong tidal effects, just as the moon’s gravity causes high and low tide on earth. Eventually, this causes celestial bodies to lose orbital energy, and they will have shorter orbital periods of only a few days.’ In other cases there may even be collisions, such as between a planet and its star or black holes colliding with each other.

### Hot Jupiters

In astronomical jargon, the process by which long, oval orbits change into close orbits is called ‘high-eccentricity migration’. According to some astronomers, this phenomenon explains the creation of so-called ‘hot Jupiters’. These are large gas giants that look a little like the planet Jupiter in our solar system, although they are much hotter as they circle their star in a very tight orbit: about a hundred times closer than our Jupiter is to the sun. Hamers’ calculations show, however, that high-eccentricity migration in planet systems with three to five planets can explain at most only 1 percent of the observed hot Jupiters. It is therefore probable that the hot Jupiters are formed mainly in other ways.

### Chaotic

Hamers’ findings also have repercussions for the way we look at our own solar system. Hamers: ‘For instance, because of measurement inaccuracies, we don’t know the exact position of a planet such as Mercury. That’s important, because the smallest difference in the starting situation can mean that a planet ends up in a completely different orbit than was predicted. The solar system is actually a chaotic system. There’s even a small chance that Mercury will collide with the sun in five billion years.’ While Hamers thinks that the earth will then remain in a stable orbit, this is really only small consolation: around the same time, the sun will grow into a ‘red giant’ and completely engulf the earth.