Hacker News Reader

Comment by luchak | Hacker News Reader

"Based on its density, scientists think GJ 1214b is made up of about three-quarters liquid water with a solid core of iron and nickel and an atmosphere of hydrogen and helium—not unlike Earth."

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.

View on HN

in theory, that sounds a lot more like Earth than any other known celestial body.

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

I'm not so sure, given the article as written. Comparing Earth and Jupiter:

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.

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-...

Actually it'd be more like a small Neptune/Uranus than a large Earth. Those two planets are made up mostly of a water/ammonia/methane mixture with a solid core and a H/He outer atmosphere.

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

Mars and Venus are more similar to Earth than this.

How do they know what it's made of? How do they even know the density?

The gist: spectrum analysis of light that passes through the planet's atmosphere from the system's sun compared to light that doesn't pass through the planet's atmosphere. They look at what light bands the planet's atmosphere absorbed. (IANA astrophysicist, learn more at http://en.wikipedia.org/wiki/Absorption_spectrum )

From what I understand though there are some assumptions made based off the readings to arrive at the description we have? As in the same spectrum differences could mean more than one thing?

I thought the article said they looked at how the stars light was dimmed by the planet. They also managed to do a spectrum analysis?

No, they didn't -- getting the absorption spectrum of a transiting exoplanet is an order of magnitude or two beyond the capabilities of present-day instruments. They just measured the density.

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.

So how do they know the mass?

This can be obtained by looking at the red/blue-shift of the star as it wobbles back and forth due to the planet's motion. Combine this with the mass of the star, and you can figure out the mass of the planet.

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

They do spectroscopy - http://en.wikipedia.org/wiki/Spectroscopy

They haven't yet done spectroscopy on it. It's mentioned as a future possibility, but right now it's not feasible to do spectroscopy on an exoplanetary atmosphere -- the signal-to-noise you get when you try to measure something as tiny as the effect of a transiting planet's atmosphere on the spectrum of a star is too damn small to be measured with today's instruments. (I just happen to have heard a talk on this very subject a few weeks ago).

Maybe in a few decades, though.

To find out what it's made of, they are thinking of using spectroscopy.

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.

Couldn't any mass planet be going any speed at any distance around a star? I'm not sure why these factors would depend on mass.

You're correct -- the orbital speed and path tell you nothing about the mass. A grain of dust will follow exactly the same orbital path as, say, Jupiter.

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

Oh so when the planet is between us and its star, it pulls the star towards us which blueshifts the light?

That sounds hard to detect.

> 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.

hugh_ is right. It's a mass/volume calculation.

Reply on news.ycombinator.com