So as an astronomer when I look at the sky with other people they always ask me what is your favorite place in the universe（ 宇宙）? What is your favorite galaxy（星系）? What is your favorite planet? My answer is Earth. That's right. This is a very special place even for an astronomer.

  We look at a lot of places but there's only one that we know of in the whole universe that we can live on. It's an amazing planet. There is an amazing number of things happening on it. Some of them you are hearing about it today. That's the only place in the universe where we know that there is life. So that makes it extremely special.

  What I'm going to talk to you about is this great adventure in astronomy that is happening where we are actually actively（活跃地） looking for other places like this. It's impossible to imagine the number of possibilities what happens on those other planets that can be habitable. So that's what I'm going to tell you about. So the first thing we have to think of is well what makes a planet habitable. And the easiest thing to do is to look at our own solar（太阳的） system. We have multiple（多重的） examples.

  The first thing we learn is that size matters. We can't have a planet that's too small or too big. If we look at a planet that's too small it doesn't have an atmosphere（大气）. The moon, technically（技术上） not a planet, but a good example for this is too small. It doesn't hold an atmosphere. Jupiter（[天] 木星）, very very big and it actually is mostly composed（组成） of gas.

  It has no surface（表面） you can stand on. The Earth is just right. The second thing that we learn is that the planet has to be at the right distance from its star. If the planet is too close to its star it's too hot. That's the case for Venus（[天] 金星）. Here I have a picture that was taken by a spacecraft（航天器） that landed on Venus（[天] 金星） and the surface（表面） although it's rocky and quite familiar to us compared to Earth it's really too hot.

  The opposite and if the planet is too far from the star it is too cold. That's the case for Mars. So we need to look for planets that are at the right distance from their star and also of the right size. So one of the things you know you might think oh this is really hard because the planet has to be just right. It only happened once in our star system but when you look at the sky at night and here is a video that I took actually from Hawaii（夏威夷）, a dark place where you can see a lot of stars, the first thing you notice is there are a lot of stars.