# How do we know the density of some extrasolar planets?

*Hello, How do you know how the weight of a very distant planet? How do they know this new plant would float on water. Is this science or just guessing?*

Weighing planets is a tricky business. We can't even see the vast majority of extrasolar planets! For most of them, all we can do is watch how their gravity makes their star wobble. But astronomers can decipher that information and use it to estimate the orbit. The problem is, we can't tell whether the orbit of the planet is face-on or edge on in the sky, so there is always some uncertainty.

Every once in a while, though, we get lucky and discover a planet that passes in front of its star, so it blocks a little of the light. This is called a "transit", and tells us a lot more about the planet. For one, we know its orbit exactly, since we know that we are seeing it exactly edge on (otherwise we wouldn't see the planet go in front of the star). If we know the orbit exactly, then we can figure out what the planet's mass is compared to the star. We can get a good estimate of the star's mass based on its brightness and color, which lets us figure out the planet's mass too.

But the statement that a planet is "light enough to float on water" implies that we know the planet's density, not just the mass. To figure out the density, we need to know the size and the mass. Luckily, when a planet transits, we can watch how long it takes the star's light to dim, and figure out how big the planet is.

So, it's not guessing when you hear astronomers say that there is a planet out there that is the largest planet ever discovered, but only weighs half as much as Jupiter. The guessing comes in when we start trying to figure out why this planet is so huge. That is still up for debate...

# Still Curious?

**Get More 'Curious?' with Our New PODCAST:**

- Podcast? Subscribe? Tell me about the Ask an Astronomer Podcast
- Subscribe to our Podcast | Listen to our current Episode
- Cool! But I can't now. Send me a quick reminder now for later.

**Related questions:**

- How do you measure a planet's mass?
- How do we know the mass of the Earth and the Moon?
- How are planets detected around other stars?

**More questions about Extrasolar Planets:** Previous | Next

# How to ask a question:

If you have a follow-up question concerning the above subject, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.

Main Page | About Us | For Teachers | Astronomy Links | Ask a Question | View a Random Question | Our Podcast

Table 'curious.Referrers' doesn't existTable 'curious.Referrers' doesn't existURL: http://curious.astro.cornell.edu/question.php?number=710

This page has been accessed *16339* times since September 29, 2006.

Last modified: *September 29, 2006 10:17:00 PM*

Ask an Astronomer is hosted by the Astronomy Department at Cornell University and is produced with PHP and MySQL.

**Warning: Your browser is misbehaving! This page
might look ugly.** (Details)