Thursday, May 17, 2007

Hot ice

If this planet doesn't work out, maybe we can go to another one. Last month astronomers discovered a planet around a star about 30 light years away that was in the habitable band where scientists would expect to find water. Seeing as how the planet was something like twice the size of Earth and rocky (i.e. not a gas giant) it promises to be the closest planet to Earth, habitat wise, of any of the extrasolar planets found to date.

Recently astronomers found another planet, but we wouldn't want to live on it. The surface temperature is around 300°C... and it's made of ice.

The planet was found back in 2004 circling a star called GJ 436 about 30 light years from here. At the time they thought it was about the size of Neptune and rocky.

Scientists found the planet by looking for a wobble in nearby stars. Much like the way the moon pulls at Earth's water causing the rising and falling tides, a planet pulls at a star causing it to wobble. That's how the planet was found. Recently, though, they've been able to watch the dimming of the star as the planet passes between it and us. This gave them more information. It told them that the planet was half the size of Neptune. Now, Neptune is mostly made of hydrogen (but with an ice core). This planet was smaller than they thought, so it had to be more dense. But it wasn't small enough to be rocky. Based on this information, they think the planet is made of ice. It's close enough to its star that the surface temperature is around 300°C or about 570°F.

How can it be so hot and covered in ice?

There are two basic ways to make a solid out of a gas or liquid. One is to cool it. Molecules in a gas bounce around, and bounce into one another. Slow the molecules down by sucking out the energy and they will eventually slow down enough to capture each other. This causes them to bond. In the case of water vapour, if you cool it down it becomes liquid water (pools together into a liquid but it's easy to split apart), and cool it down some more it becomes a solid (the molecules in the water stick together and form a crystal structure we all know as ice).

The other way to form a solid is to push the molecules together with enough pressure that they can't help but bounce into each other. Press them together with even more pressure and they can't bounce off each other; they are forced to stick together and form a solid. One of the properties we like so much about water is that it's difficult to compress. If you put water in a container and push it you can't easily compress it. Instead, it presses out with the same force with which you push it. This is the basis of hydraulics. It takes an awful lot of pressure to turn liquid water into a solid, but it can be done. Push water molecules together hard enough and they will bind into each other, just like carbon molecules in the air if pressed together hard enough will form coal or a diamond.

This planet is not dense enough to be solid rock, so scientists speculate that it's a special form of ice created from a combination of the planet's own gravity and the weight of its atmosphere. This solid water, such as Ice VII and Ice X, can exist even at higher temperatures. The atmosphere would be basically steam, with a core of ice. Because the water would have boiled off early on, given the planet's proximity to its star, scientists think it was created further away from the star but was somehow pulled into a closer orbit.

For more information, here's the article:

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