Found: One Hot and Rocky Little Exoplanet

February 03, 2009

Ladies and gents, we've found the smallest-ever rocky planet outside of the solar system -- way outside of the solar system. (About 456 light-years outside, to be precise.)

Nevermind that COROT-Exo-7b is likely a place covered in molten lava and/or face-melting steam; it's still rocky and we know it (clap your hands)!

This from the press release:

"For the first time, we have unambiguously detected a planet that is 'rocky' in the same sense as our own Earth." - Malcolm Fridlund, ESA's COROT (convection rotation and transits) project scientist

This brand-new exoplanet is about half two times the diameter of Earth.*

Since hardly any of us can remember the precise diameters of solar system bodies, I whipped up this graphic for a sense of scale (I make no warranties that this is exact, but it should be close):

Ok, a big white circle + question mark = rather lame for visualizing a scorching-hot rocky exoplanet.

Try this speculative graphic on for size (still image borrowed from this frightening YouTube video):

I know you have questions -- I did -- so let's see if we can knock those out:

1. Why is this planet so smokin' hot?
2. What's up with all of the steam?
3. How can we figure this out from 456 light-years away?

1) Its year is only 20 hours. Compare that to Earth's year, which lasts 365 days. To complete an orbit in less than one Earth day means this exoplanet has to be extremely close to its parent star (a high-five to Kepler for figuring out this rule in the 1500s).

2) About that steam... Scientists think COROT-Exo-7b might be -- by weight -- about half rock, half water. Sounds surprising, right? But consider that adding a little hydrogen gas and a lot of heat to moon dust produces water. (This probably isn't what's going on with the new planet, but it goes to show that rocks can be turned into water. Whodathunkit?)

3) We know all of this because the COROT spacecraft sees planets moving in front of their star(s).
With a good set of images, you can estimate a planet's size.
Once you use the images to figure out that planet's orbit, too, you can guess its weight.
With that rough size and orbit you can also estimate the planet's density -- solid? Liquid? Both? In this case, it seems to be either molten lava or half water/half rock.

In any case, it's a scorching 1,800-2,700 degrees F (1,000-1,500 degrees C). Ouch!

Now get this: Sometimes telescopes can capture a tiny bit of light coming off of an exoplanet. Doesn't look like that's the case with COROT-Exo-7b, but when that does happen...

Enter the amazing world of spectroscopy!

Those stray photons carry with them priceless information that can tell us what, exactly, the planet is made of and how hot it is. This is, by the way, how we know most of what we do about distant stars and other exoplanets.

Examples? Below is what hydrogen looks like via a spectroscope:

 

Here's iron:

And the sun:

Hopefully we'll be able to get such a spectrum out of this new hot, steamy, and rocky little exoplanet.

But until then: The next time you're at the pub, be the life of the party by raising your glass to emission and absorption spectra. I promise no one will make fun of you.**

Images, top to bottom: NASA/Dave Mosher; NASA/YouTube/Dave Mosher; Wikipedia; Wikipedia; Wikipedia N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF***

Fodder: ESA

* A big thank you Casey10s on Phil Plait's Bad Astronomy blog for pointing out this hideous mistake!
** This is a lie.
*** Thanks to MadScientist for the credit correction.

Comments

[شات]

nice
thanks

[Dave Mosher]

@andy Ok, we're getting into semantics here and that's ok. It's a big deal in astronomy these days :)
Personally, I wouldn't consider a body orbiting a pulsar a traditional planet. Most of these are the rocky cores of failed stars/gas giants with their gas blown off, aka "chthonian" planets. I think they're super cool to think about, e.g. they'd be as close to hell as you could get, being blasted continuously by a furiously spinning neutron star.
Now by "rocky planet" I mean (and I'm pretty sure the astronomers do too) one formed into a rocky body by accretion at the outset, such as the Earth. So perhaps I should have said: Smallest Rocky Planet That's Kind of Like the Earth.
But that's quite a lot of text to put into a headline!

[andy]

Claiming it is the smallest rocky planet is somewhat dubious ... the smallest known exoplanet is the pulsar planet PSR B1257+12A which has a mass comparable to the moon, far too small to be a gas giant.

[Birdman]

To me I thought gliese 581c was a bit more promising cuz it could have liquid water present and isn't as hot but I'm not an expert I just like to read what the experts have to say.

[Dave Mosher]

@chrl It indeed is Australia! It's a still I grabbed from a great, HD-quality YouTube video showing Earth getting whacked by a big asteroid (check the link about the second pic).

@Kumar If COROT-Exo-7b can get a few millions miles away from its star, then the chances might be slightly better. Otherwise it's going to be toasty for a long time!

@Grammarwitch Thanks! I'd love teach in a different life, but who knows...

[Grammarwitch]

I'm with your mom on the Mars thing, but if you ever get tired of the Discovery Channel and yearn for a job that pays almost nothing, you'd make an amazing teacher. If you brought the same sense of humor and accessible explanations to the classroom that you brought to this article, kids would be lining up. And, unlike most science education nowadays, they'd actually GET it. Think about it.

[Kumar]

I like this planet...cuz after millions of years.. if if life forms on this planet it can support huge population.. lol

[chrl]

On the exoplanet that little patch of rock looks like australia LOL

[Zooplah]

I like the second picture. It looks like the Genesis planet forming.

[Mr Science30]

wow this corot thing sounds really cool. im wondering what it will tell us

[michael]

This article on the exoplanet corot (whatever the rest of it's name was) is very interesting. And I love the sense of humor that comes with it.
Discovery channel rocks!!!

[Allen Smilden]

Hi again, thanks for the graphic. It is appreciated. :)

[Dave Mosher]

@MadScientist:

Thanks very much for the link -- credit line fixed! Yet another reminder to never trust a Wikipedia image (will us new media types ever learn?)

@Allen Smilden:

You're *absolutely* correct about my goof with the 2X vs. 0.5X diameter -- I made a big mistake. Zeus knows how many times I read the release, but the "less than" before "twice the diameter of" threw me in a loop. My brain was convinced that it meant 1/2. D'oh.

At any rate, the post is fixed and I will eternally slap my forehead.

[Allen Smilden]

According Esa's website the diameter of this planet is less than twice that of the Earth's. Which would make it appear larger than the Earth.

http://www.esa.int/esaCP/SEM7G6XPXPF_index_0.html

[MadScientist]

Uh - you need to work on your attributions. That synthesised solar spectrum was produced from data obtained at the Kitt Peak Solar Observatory; don't cite the wikipedia, the proper source is here:

http://www.noao.edu/image_gallery/html/im0600.html

In fact, if you look at the 'conditions of use', quoting the Wikipedia violates those conditions:

"Minimum credit line: N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF"