Dry, hot, dusty alien worlds

By Phil Plait | February 21, 2007 11:26 am

Update (Feb. 21, 2007): Two papers are online describing these results. They’re technical, but if you’re interested: one on HD 189733b and one on HD 209458b.

Astronomers using the orbiting Spitzer Space Telescope have, for the first time, seen the emission spectrum from planets orbiting another star! This is extremely cool, as you’ll see.

First off, a spectrum is what you get when you break up light into its individual colors. Think of it like a rainbow, but with a lot more colors. By carefully examining the brightness of each color, you can tell a lot about the object giving off that light — for example, its temperature, its chemical composition, and even how it’s moving. In this case, astronomers used Spitzer to examine the spectra of two planets in the infrared, light invisible to the naked eye. Many interesting molecules emit infrared light, so it’s a good wavelength to investigate.

Spitzer was designed to observe in the IR, so a team of astronomers pointed Spitzer at two stars: HD 209458 (about 150 light years away in the constellation of Pegasus), and HD 189733 (about 60 light years away in the constellation of in Vulpecula). These two stars are known to have giant planets orbiting them. The planets are very close to their stars: they orbit at 4 million and 3 million miles out, respectively– for comparison, the Earth is 93 million miles from the Sun, so these planets are really close to the stars! The orientation of the planet’ orbits is such that every orbit they pass behind their parent stars, creating a planetary eclipse. This allowed the astronomers to use a technique that reveals the planets’ spectra.

The astronomers observed the stars when the planets were hidden from view, so they got spectra of just the stars. Then they repeated the observations when the planets were next to their stars, yielding spectra of the two together (from Earth, the planet and star appear as a single point of light because they are so far away from us). Once they have both spectra, they can carefully subtract the light from the star form the combined star and planet, leaving just the planet: star + planet – star = planet. Bang! Done.

Well, it’s not that easy. I’ve worked on this kind of observation myself, and it’s incredibly painstaking and detailed. It takes a huge amount of work, and my hat is off to these guys who did this observation.


So what did they see? Perhaps the biggest thing is what they didn’t see: water! Theoretically, water (actually, steam; these planets are hot) should be present in the planets’ atmospheres, and should have been really obvious. Water is excellent at absorbing infrared light (it’s a greenhouse gas, after all) and so the presence of water would have been revealed by dips in the spectra at specific wavelengths. They saw nothing like that!

Does this mean there is no water in the planets’ air? Not necessarily; more likely the water is lower down in the atmosphere, and is covered by high altitude dry clouds of some other chemical, and the light is blocked.

This is supported by the detection of silicate dust in the atmospheres. This means the planets have a high layer of dust surrounding them, unlike any planet in our own solar system. So that’s pretty interesting as well. Not only that, they found a feature in the spectrum at 7.8 microns they can’t identify! That’s unusual, and they need to take more observations to try to nail that down.

One more thing, and to me it’s the most fascinating result they found. The shape of the spectrum can tell you a lot as well. These planets are so close to their parent stars that they are tidally locked, meaning they always show one face to their star, like the Moon does to the Earth. You might expect that one side of the planet gets incredibly hot while the dark side gets cold. Well, that can be determined by looking carefully at the shape of the planet’s spectrum. What they found was that the heat from the day side is somehow being transported to the dark side. The easiest way to do this is through winds.

So what we have here are planets more massive than Jupiter yet only a few million miles from their stars. They have one face locked toward their star, and the other forever facing away. They have deeply buried clouds of steam, high altitude bands of dust, and a lot of wind which whips around the planets, warming up the night sides and keeping the atmosphere from literally freezing out.

You know, we have names for these objects like "extrasolar planets", and "exoplanets". But maybe we should call them as they are: alien worlds.

But now, because we’re smart, and because we want to learn, they won’t stay alien for long. This is the first time in history we’ve been able to observe alien planets in this way, but it’s not the last. We’ll find more, and we’ll learn more. We always do.

CATEGORIZED UNDER: Astronomy, Cool stuff, NASA, Science

Comments (40)

  1. J.Serrano

    Wow, even I could follow the reasoning of how the bright guys have learnt about the composition of an object we cannot see. And since you were using miles in a comparative way (3 or 4 million against 93) it didn’t bothered me at all. (I usually get lost when given distances in miles or inches).

    Thanks for great explanation in so plain terms BA.

  2. “Not only that, they found a feature in the spectrum at 7.8 microns they can’t identify!”

    Man, that’s such a science fiction statement! This is what happens in the movie just before the guys in the black suits show up to close down the lab and murder the scientist to cover up the Big Alien Conspiracy.

    Living in The Future is awesome!

  3. Gary Ansorge

    I look forward to the day we can build REALLY BIG telescopes in orbit, big enough to collect detailed images of some of these alien worlds.
    I hope someday we have a solar system spanning civilization that has the wealth(ie,energy and other material resources) to send exploratory craft to these alien worlds and really check them out.

    Darn, so much to know. So little time to get to know it,,,that’s the only reason I have to wish to live a really long time.

    Gary 7

  4. Quiet_Desperation

    >>>they found a feature in the spectrum at 7.8 microns they can’t identify!

    Clearly it’s a vast patch of Naquada. Or Unobtainium. Upsidaisyium?

  5. wright

    Wow.

    Just… wow.

    That we can discover so much about worlds that are so incredibly distant, and so enticingly strange. What a time to be alive, when such knowledge is pouring into public venues like the internet.

    A great job of explaining the basic science used, Phil. Please keep it up.

  6. The Dread Polack

    Awesome. This is just the sort of thing in astronomy that gets me really excited. How anyone would rather hear about Anna Nicole’s baby than frickin’ ALIEN WORLDS, I will never understand.

  7. ioresult

    BA said: “It takes a huge amount of work”.
    Not to undermine the incredible work they did, but… what is hard in this? Let me summarize the steps:
    -Get time on ‘scope
    -Point scope towards star
    -Time brightness dips in order to know when the planet passes behind star
    -Take spectrum when planet behind star
    -Take spectrum when planet not behind star
    -Perform vector subtraction
    -Publish paper with numbers

    The hardest part for me would be to get some ‘scope time…

  8. The BA says in the setup: “So what did they see? Perhaps the biggest thing is what they didn’t see: water!”

    …and then in the conclusion: “So what we have here are planets [with]…deeply buried clouds of steam…”

    Evidence, Phil, evidence. Yes, the model says you SHOULD see water, but there is no evidence of it in the data.

    Still, a mind-boggling entry!

    – Jack

  9. Do I call Virgin Galactic to book passage?

  10. This is supported by the detection of silicate dust in the atmospheres. This means the planets have a high layer of dust surrounding them, unlike any planet in our own solar system. So that’s pretty interesting as well. Not only that, they found a feature in the spectrum at 7.8 microns they can’t identify! That’s unusual, and they need to take more observations to try to nail that down.

    They found Dune. I’ll bet anybody $100 that the unknown feature is Spice.

  11. Justin

    Got to be Kryptonite! :)

  12. Scott

    “I look forward to the day we can build REALLY BIG telescopes in orbit…”

    I think we’re starting to realize we’re better off with multiple smaller satellites than one giant one. One reason being that they can correct each other’s vision—if it sees something another does not we’re not mislead.

    I look forward to a whole fleet of orbiting Hubbles and Sptizers.

  13. Kullat Nunu

    So what we have here are planets more massive than Jupiter yet only a few million miles from their stars.

    Well, HD 189733b is slightly more massive than Jupiter, but HD 209458b is much less massive (0.69 times Jupiter, more typical value for a nearby transiting planet).

    Something’s not right with HD 209458b, however. It is much larger than it is supposed to be (about 1.3 times as large as Jupiter), much bigger than could be explained by external warming only. Still, there are several other puffed-up planets, so obviously it isn’t an unusual phenomenon.

  14. I have to say that I also dislike the term “extrasolar planet”. Earth is extrasolar. I’d hate to see what life would be like if it were inside the Sun.

  15. Kullat Nunu

    I look forward to a whole fleet of orbiting Hubbles and Sptizers.

    There already is a whole fleet of orbiting Hubbles in the space–they just look in the wrong way! Hubble is based on spy satellites, some of which obviously have quite formidable primary mirrors.

  16. Kullat Nunu

    I think we’re starting to realize we’re better off with multiple smaller satellites than one giant one.

    There are many things that can’t be done with small telescopes.

    Fortunately, there is also a lot that can be done with tiny scopes, take for example COROT, which may find the first transiting hot Neptunes, and possibly even the first transiting super-Earths.

    Even the puniest of puny, 15 cm MOST telescope has been used successfully in extrasolar planet science. It detected that the outer layers of Tau Boötis rotate at the same speed as the planet b orbits the star.

    If only we had more of them… money can’t be an overwhelming issue since they cost only a tiny fraction of a large telescope.

  17. Chip

    Thank you for yet another cool story!

    P.S. I dig “Artist’s Conception Planets” – always fun to see – but maybe someday we will actually glimpse one of these worlds in addition to the numbers telling us they are there.

  18. Gary Mcleod

    The first alien world spectograph, amazing! I just wish NASA would send up the Terrestrial Planet Finder network of satellites that would be able to obtain actual visual images of these extra-solar planets, like, NOW!

  19. Good points in the comments.

    I should have said “about Jupiter sized” to be more generic.

    About water: it is almost certainly there; we see it all over the place in our solar system, and the conditions are good for it in these planets too. It’s possible it’s not there, but I would bet it’s just down deeper.

    About getting the spectra: yes, it’s hard. Subtracting the star ight sounds easy, but in practice it’s a monumental task. It’s thousands of times brighter than the planet, even at these wavelengths. I spent many weeks building models of observations of a Jupiter-sized planet orbiting Alpha Centauri at 5 AU, and it turned out to not be possible with our camera (though just barely, dangit). But I also worked very hard on trying to extract an object’s spectrum from its parent star’s spectrum, and we spent weeks on it, and eventually gave up. It was impossible due to scattered light. That was heartbreaking.

  20. Kullat Nunu Says: “There already is a whole fleet of orbiting Hubbles in the space–they just look in the wrong way! Hubble is based on spy satellites, some of which obviously have quite formidable primary mirrors.”

    SHHHHhhhhh!

    – Jack

  21. The Bad Astronomer Says: “About water: it is almost certainly there; we see it all over the place in our solar system, and the conditions are good for it in these planets too. It’s possible it’s not there, but I would bet it’s just down deeper.”

    Right, that’s why I said “the model says you should see it” and if I were the betting type, that’s the way I’d go. But we don’t see the evidence yet. Look how much trouble we’re having verifying that there’s water on the moon!

    – Jack

  22. OtherRob

    Yes, but where does astrology fit in all this?

    Joking, joking! Put down the pitchforks!

  23. Tim G

    I spent many weeks building models of observations of a Jupiter-sized planet orbiting Alpha Centauri at 5 AU

    Are you talking about the red dwarf or one of the two inner stars of the Alpha Centauri system? A planet wouldn’t have a stable orbit at that distance around one of the brighter two stars due to perturbations by the companion.

    Anyway, it’s a crime that congress won’t secure funding for the Terrestrial Planet Finder. That mission would allow the detection of earth-like planets. Earth wouldn’t need to be near the plane of the alien planet’s orbit.

  24. Tony Webster

    I am not a scientist – I lack the both painstaking ability that the analysis of spectra requires and the ability to see both the links and the gaps in the chain of evidence – I can only say that it is this kind of article of yours that makes me wish I could be what I am not!
    I know why you call the site the Bad Astronomy Blog but, in truth, it should be the Good Astronomy Blog.
    In reference to your comments about religion this stuff is about awe and wonder related both to the fact that such worlds exist and to the fact that there are those amongst us who can and will find out more.

  25. Grand Lunar

    “….Explore strange new worlds….”

    I know it’s cliche, but that’s what comes to mind.

    I find it odd just how many “hot Jupiters” we are finding. It makes me wonder if this is the norm, and our own solar system is an oddball.

    Anyway, this type of study is only the beginning! I wonder what further discoveries will be made.

  26. Kullat Nunu

    “Today’s Most Stupid Title” award goes to CBS —

    CBS: NASA Identifies Possible Life Outside Solar System

    Link: http://cbs2.com/topstories/local_story_052155509.html

  27. Tim G

    Kullat,

    Perhaps Katie Couric is having an influence on CBS news.

  28. Samara

    Yay! In these days of looming budget cuts for NASA and anti-science running amok, it’s nice to hear that scientists are still quietly going on with their work and accomplishing something.

    Just curious. Could this system be used to detect whether there is life on planets? Possibly detecting gases or something that can only be produced by life (like methane)?

  29. Gary Ansorge

    Grand Lunar:
    Hot, Jovian size planets are close to their suns, and therefore induce larger solar perturbations which are easier to detect.

    Distant Jovians induce much smaller perturbation, so are harder to detect.
    Am I in the ball park with that, BA?

    Gary 7

  30. icemith

    Kullat Nunu (at 5:32 am): I thank you for the link to that absolutely in-ept piece of journalism (?) by CBS. Whatever were they thinking about? Certainly not Science. Almost nothing quoted made sense!

    First, why quote interstellar distances in miles?. Even Kilometers is totally in-appropriate! Astronomers are more likely to use lightyears’ and that is a self- explanatory term. They could have researched that instead of saying “many lightyears”.

    But the doosie is the lack of sense in the last two paragraphs:

    “In previous observations of HD 209458b, NASA’s Hubble Space Telescope measured the light from the star, not the planet. Those observations showed elements like sodium, oxygen, carbon and hydrogen, which bounced around the top of the planet.

    “This time, researchers used the “secondary eclipse” technique to detect light from the planets. Using that method, the telescope monitored the planet as it circled behind its star, temporarily disappearing from view.”

    I’m glad BA was able to translate that example (thanks Phil for being so informative), as what they said was gobblygook. If that was evidence in a courtroom, I think the Judge would throw the book at them. (Or do we have another ID mole in the Publicity Dept of NASA?

    I hope not.

    Ivan.

  31. John

    ” star + planet – star = planet. Bang! Done.”

    Very, very cool.

    “P.S. I dig “Artist’s Conception Planets” – always fun to see – but maybe someday we will actually glimpse one of these worlds in addition to the numbers telling us they are there.”

    That’s not the actual picture?! Dang. (j/k)

    So, Mercury’s orbit is about 35 million miles, right? These guys are about a ninth of that distance? Incredible. I would have thought being that close would strip the atmosphere away.

  32. Irishman

    Samara, if you read the really poorly titled article linked by Kallat Nunu, you will see they discuss the ramifications for using this technique to look for life.

  33. Gary Ansorge

    John:
    Eventually, the atmosphere will be blown away. But it’ll take a while(ok, I have no idea how long, but probably in the millions of years, rather than billions,,,)

    What interests me is how such a body could even form so close to it’s primary star. I would have thought the intensity of solar radiation would blow light gases away before they could condense into a planet???

    GAry 7

  34. Great catch Kullat! You are now immortalized in blogdom. :-)

    By the way, I listened in on the press conference, and one of the scientists gave the distances to the two stars in miles. I thought that was funny; I had to translate one into light years for the blog. ;-)

    And that second-to-last paragraph in the story is weird. I have no idea what they mean by that.

  35. Kullat Nunu

    Thanks BA, this is almost as funny as getting my (real) name in an AAS poster by writing only one article in the the Slackerpedia Galactica! :D

    (Actually, I didn’t find that gem, blame Greg Laughlin for that; see his systemic blog @ http://oklo.org/?p=191 He’s got more about the findings.)

  36. David Semloh

    I contacted Phil on this very subject about 7 years ago. He even referred a colleague to me, who never replied.

    Perhaps more important is the possiblility of terraforming Mercury, which would keep a significant atmosphere for at least 40 million years it seems. If water is transported, the inevitable or even quick dissociation could make a breathable atmosphere with a risidual 02 base. Unfortunately, N2 is much harder to source.

    The top post is somewhat incorrect. Mercury for example is not tidally locked, and a large Jupiter scale planet is likely to be not completely locked even with being only 3 million miles from the star or so I would think . Smaller bodies lose their angular momentum quicker I recall and the same is true for rotation in tidal senses. The lunar moon, sister planet or not, is still only 1:1,000 ratio of Earth mass and was originally many pieces after the breakoff.

    Of course any Jupiter style planet would be about the same ratio or less with a stellar object mass and the extra distance would be nulled by the greater gravity, perhaps. But much of being tidally locked could be due to previous hits with other celestial objects.

    Finally, dust in a Jupiter type of planet might only come from a ring that is steadily slipping passed the roche limit. Such planets do not have a real surface as we know it. The silicate and metalic is moulton and hydrogen perhaps metalic under the pressure. And the mix rate is very little, especially to the cloud levels.

    A Venus style planet, with 250 atmospheres could, since the insolation is so great. Only about 2% of the incoming radiation hit the surface, but at 3 million miles the rate is great enough.

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