what

This is either terrible writing or terrible science (on the author's part, not the scientists', I would imagine). About 70% of the Earth's surface is water, but only a fraction of a percent of Earth's mass is water. (http://bit.ly/7Gfk8V, total water volume from http://en.wikipedia.org/wiki/Water#Water_on_Earth). And the Earth's atmosphere is definitely not primarily hydrogen and helium.

now, whether or not it's all accurate, we'll find out eventually.

1. made up of about three-quarters liquid water

False for both. Possibly true for Earth if you're talking about surface area, but that's a pretty generous interpretation of the article as written.

2. with a solid core of iron and nickel

Likely true for both. ("At the center of [Jupiter's] rocky core is probably a metallic ball of iron and nickel, just like Earth's core." http://www.spacedaily.com/reports/Jupiter_Has_Large_Rocky_Co...)

3. an atmosphere of hydrogen and helium

True for Jupiter, not for Earth.

That's 2 of 3 for Jupiter, 1 of 3 for Earth (or 2 of 3 for Earth if you're being generous). Mars sounds a lot more like Earth than GJ 1214b does, based on the article.

GJ 1214b might be more like Earth than any other known exoplanet, but given the murky reporting it's hard to tell.

I'd say the most Earthlike exoplanet discovered so far is CoRoT-7b, discovered a few months ago. It's about five times more massive than Earth, but has a similar density, implying it should consist mostly of rock.

http://spacefellowship.com/2009/09/16/smallest-exoplanet-is-...

http://en.wikipedia.org/wiki/Neptune#Composition_and_structu...

To find out the density, it's mass over volume. They infer the mass by the planet's distance from its star and by its orbiting speed, and they get the volume by the amount of light it takes away from the star when it pases in front of it.

It's the redshift/blueshift of the actual star that you need to measure.

That sounds hard to detect.

Well, obviously you need really precise spectrometers, but just because the effect is tiny, doesn't mean that you can't detect it.

From the density, though, you can get a pretty good idea of what it looks like. There are three basic ingredients which go into making planets: rock (including pure metals), "ice" (a catch-all term which includes all phases of water along with methane and ammonia) and hydrogen/helium mixture. Everything else is too rare to worry about. Knowing (basically) the density of each of these components, plus knowing the fact that H/He mixture won't accrete to a body until its mass goes beyond a certain point, you can put pretty good constraints on what the composition of a planet of a certain mass and radius should be.

Of course we could be wrong -- it could equally well be a ball of pure argon or something; however it's hard to imagine how any such thing would form.

I forget how you get the mass of the star, but I think it can be pretty well established from its brightness.

Maybe in a few decades, though.

Maybe it doesn't count if NASA doesn't build it?

http://en.wikipedia.org/wiki/Titan_Saturn_System_Mission

This one would have included a lake lander and a balloon, and would have been generally neat-o, however I agree with NASA that Europa should be a higher priority for now.