Let’s meet a planet called WASP-12b. [Before you ask, extra-solar planets are named based on how they were discovered or their catalog number. Wasp-12b is the twelfth found by the WASP (“Wide Angle Search for Planets”) survey. The “b” indicates that the object is a companion; Wasp-12 itself is the planet-hosting star.]
WASP-12b is a “hot Jupiter”. This means that it has two key features. First, it’s a planet that’s really hot because it orbits very close to its star. Second, like Jupiter it is a giant ball of mostly gas, rather than rock or iron or something else.
This plot shows the measured sizes and orbital distances of extra-solar planets. Hot Jupiters are about the same size as Jupiter — even a little bigger. But much much closer to their stars.
WASP-12b is by the left edge of the “hot Jupiter” bubble in the image. Its orbit is tiny, just 2% the size of Earth’s. In fact, its orbital radius is only three times larger than the radius of the star it orbits! The star spans almost 20 degrees on the sky as viewed from Wasp-12b! That is forty times larger than the Sun (or the full Moon) in the sky on Earth. It looks as big in the sky as a Frisbee held at arm’s length. As big as a hula hoop on the ceiling if you are lying on the floor. Scary big!
One year on WASP-12b takes just 26 hours! That is the time the planet takes to complete a full orbit around the star. The planet’s surface temperature is 2500 Kelvin (4000 Fahrenheit)! Steel would melt on the planet’s surface. In fact, there are only a handful of metals that are not melted at 2500 Kelvin (like molybdenum and osmium). This is where the “hot” from “hot Jupiter” comes from.
This is what an artist thinks Wasp-12b might look like.
It looks like WASP-12b is in a tug of war with the star. That is because the planet is in the process of being torn apart by the star’s gravity! The planet is being shredded and falling onto the star. The equivalent of the Earth falls onto the star every 30,000 years. That is like a chunk the size of Connecticut crashing down every year! Gas is drawn from the outer layers of the planet onto a disk of gas close to the star and then down to the star’s surface. The planet itself is gigantic — nearly twice Jupiter’s size — because it is basically being drawn and quartered! But instead of horses tearing it apart, it’s its own star’s gravity.
Because hot Jupiters are the easiest kind of extra-solar planets to find, they have been studied the most of any type of extra-solar planet. Hot Jupiters’ orbits are short so any signal you find is repeated within a few days. And because hot Jupiters are so massive they produce a big signal.
What do we know about hot Jupiters? We know that they are rare, Only one in every 100 to 200 stars like the Sun has one. We know that they are often extremely puffed up. Hot Jupiters with the same mass as Jupiter are typically 20%, 50%, even 100% larger than Jupiter. That is like a six-year old boy puffed up to the size of his dad. We don’t know exactly what is puffing up these planets, although it is probably related to either the star’s irradiation or the star’s gravity. Wasp-12b is a rare case in that we are confident that gravity is the culprit.
Some hot Jupiters transit their stars. These planets’ orbits pass between our telescope and their star. The light from the star therefore dips once every orbit:
We can learn a lot from transiting hot Jupiters. First and foremost, we measure planets’ sizes from the dip in the star’s brightness: a larger planet makes a bigger dip. Second, during an orbit the planet will go through a full set of phases as viewed from Earth. During transit we only see the dark unlit side of the planet. During the rest of its orbit the planet goes from a crescent to fully-illuminated and back. By cleverly analyzing the brightness of the star over a hot Jupiter orbit it is possible to map the surface of the planet! Third, we can learn something about the planet’s atmosphere. We know when a transiting planet passes in front of and behind its star. When the planet passes in front of the star, a fraction of the starlight passes through the planet’s atmosphere. This leaves fingerprints of the atmosphere’s composition in the star’s spectrum. When the planet passes behind the star we observe the star but not the planet. If we subtract an observation of just the star from an observation when the planet is in the picture then we can isolate the planet’s light and gather information on the planet’s temperature and atmosphere. These techniques are very difficult but very promising.
Cleverly-timed observations of stars with transiting planets can also give a 3-D view of the planet’s orbit. This is called the Rossiter-McLaughlin effect. It is spectacularly cool and will be featured in a future post so I won’t go into detail on it here.
In the Solar System, gas giants are far from the Sun and small rocky planets are closer-in. Hot Jupiters are different; they show us that not all planetary systems are as well laid-out as the Solar System. That is weird, and, honestly, it makes people like me who study planet formation look bad. But let’s put aside our wounded pride for a moment and ask: Where do hot Jupiters come from? That will be the subject of a future post.
Next week we will boldly fly into the realm of hot super-Earths!
Questions, comments, words of wisdom?