We’ve discussed some classes of planet that are pretty strange: hot super-Earths, hot Jupiters and eccentric gas giants. But that was just the warm-up. In this post we’ll check out three more extra-weird ones.
First up: Tatoine planets. In case you are less nerdy than you should be, Tatooine is where Luke came from in Star Wars. Instead of orbiting single stars like the Sun, Tatooine planets orbit in double (also called “binary”) star systems. Their orbits take them around two stars that orbit each other very fast. The Kepler mission has found a few of these systems. The most interesting one found so far is Kepler-47.
Kepler-47 is made up of two stars that orbit each other about once a week. One star is similar to the Sun and the other is a small faint “red dwarf”. The Kepler-47 system has two planets. Each is about the size of Neptune. The inner one takes about 50 days to orbit around the pair of stars and the outer one takes ten months. The outer planet is located in the habitable zone, where the temperature is about right for water to exist on its surface. Unfortunately, the planet is gaseous and has no surface. Might it have a moon that could host life?
Up next: planets on very long orbits. Stars are much brighter than planets. Stars wash out the light from close-in planets, so astronomers generally use indirect techniques to find and study planets. But if the planets are far enough away from their stars, they can block the starlight and see the planet directly. This has been done successfully in a few cases.
HR 8799 is our next weirdo. It has four known planets on very wide orbits. Each is a gas giant five to ten times more massive than Jupiter.
Imagine each star is a city. Earth is a small house in the suburbs. Uranus and Neptune are pretty big houses on a country road 20 miles away. The HR 8799 planets are huge mansions way out of town off some tiny overgrown dirt road in the middle of nowhere.
The HR8799 system is full of surprises. The three outer planets may be in a special orbital configuration called a resonance. For every orbit of the outermost planet (b), the second-outermost planet (c) completes two orbits and the third-outermost planet (d) completes four orbits. This celestial dance keeps the planets in line and stable. In fact, three of Jupiter’s moons (Io, Europa and Ganymede) are in the same resonance. So are three planets in the system Gliese 876.
The HR 8799 system contains both warm and cold dust. The cold dust is located even farther away than the outermost planet. The warm dust is located closer-in than the innermost planet. The dust temperatures are actually similar to dust in the Solar System. In the Solar System, cold dust comes from beyond Neptune, and warm dust from the asteroid belt. Around HR 8799 the dust is much farther away but has about the same temperature.
The star HR 8799 is about five times brighter than the Sun. So planets on wide orbits are not as cold as we might expect. Maybe our analogy was off. Maybe HR 8799’s “city” is a super-mega-metropolis like Tokyo or Mexico City. Everything is scaled up compared with a small-sized city. The suburbs and countryside are farther away even though they are really the same as for a smaller city. But the lay of the land is the same.
Last up: free-floating planets. These planets are not in the suburbs or the countryside. They are hobos who hop the train from town to town (or those houses that ride on flatbed trucks). Planets that wander between the stars. They still orbit within the Galaxy but are not associated with stars.
For every star in the Galaxy there are 2-3 free-floating gas giant planets! These things are everywhere! How do free-floating planets get there? One idea is that they formed in orbit around stars and were then lost to space. One way to lose planets is in dynamical instabilities, the same ones that are responsible for the stretched-out orbits of many extra-solar planets. But there are just too many free-floaters to make the numbers work. The mystery remains.
Of course, with no star there is no external source of heat. So free-floating planets must be lifeless hunks of gas and rock, right?
Not so fast! If a planet is born with enough heat (say, from a star) and has the right kind of atmosphere (with lots of hydrogen), then it can hold on to its heat pretty much forever. Liquid water might exist on an Earth-sized free-floating planet! What about life? Sure, why not? But what kind and how much? That is actually a pretty complex question that we will get back to in a later blog post….
Questions, comments, words of wisdom?