Astronomer make first map of extrasolar planet!

By Phil Plait | May 9, 2007 9:51 am
Artist’s conception of the planet HD189733b.

Wow, more cool extrasolar planet news.

Using the Spitzer Space Telescope, astronomers have been able to make, for the very first time, a (very crude) map of the super-Jupiter orbiting the star HD 189733, about 60 light years away. The planet was discovered in 2005 (note this is not the new "Earthlike" planet found recently, this is a big gas giant like Jupiter) and is one of a handful that passes directly in front of its star as seen by us on Earth. In other words, it transits the star, making a little eclipse once per orbit.

Transiting planets are very useful: by measuring how much the starlight drops, we can measure the size of the planet! The bigger the planet, the more the light from the star dims. This gives us a direct measurement of the planet’s radius.

But it also allows a cool technique to measure features on the planet, too. Imagine there is a really bright spot smack dab in the middle of the planet. As it orbits the star, the planet gets blocked by the star. We see the total system brightness drop (because the planet contributes a little bit of light). But when the bright spot gets blocked, the brightness would make a sudden dip. And if instead of a bright spot, we had a dark spot, when it got covered by the star, the brightness would not drop as quickly.

So by very carefully measuring the way the brightness changes as the planet goes behind the star, we can actually make crude maps of planetary features, even though the planet itself is far too small to resolve into a disk. This technique has been used to make maps of Pluto and its moon Charon, in fact.

But doing it for an extrasolar planet is a bit tougher! They are incredibly dim, and the stars very bright. But it’s possible, and so Heather Knutson, a graduate student (!!) at the Center for Astrophysics and her team did just that using Spitzer! Voila:

Like I said, the map is crude (yet it represents 33 hours of observations and a quarter million data points!), but it does show one obvious feature. Spitzer measures infrared light, so brighter objects in this case are warmer. That bright spot is a hot spot they found, which is roughly twice the size of Jupiter’s Great Red Spot (which itself is several times bigger than Earth!). As it happens, the planet spins once for every time it goes around the star, so it always shows the same face to its star (the same way the Moon always shows the same face to Earth). So you might expect the hottest part of the planet to be right under the star, where the star is always directly overhead. But that’s not the case; the hot spot is actually about 30 degrees away from the "substellar point". Knutson speculates that this is due to winds on the planet; a sort of alien jet stream. That sounds plausible to me.

At the moment this is the best map that can be made. But over time, as they make more observations, it is possible to improve on this map. Are there other warm spots? Are there cool spots? Can we learn more about the big spot? After all, all we know is its rough size and where it is, but not its shape. That might be possible to determine, though I can’t imagine how difficult the observations would be!

This is a fantastic step forward, and it makes me even more excited to think about what we might learn in the next few years. It takes advanced technology, but it also takes clever people to figure out how to use it to its best advantage. As it happens, we have both.


Comments (28)

  1. Tara Mobley

    That is so cool. Even a crude map is still a map of a planet outside our solar system! I hope that we’ll be able to map that Earthlike planet within my lifetime.

  2. MaW

    This kind of thing makes me very excited – and even better a friend of mine’s doing a PhD in something to do with extrasolar planet detection. Hopefully I’ll see his name on one of these wonderful discoveries one day.

  3. wright

    I’ve said it before: we live in a golden age of science. That we can accomplish observations like this and produce those capable of making them… and then have those specialists share their discoveries to the rest of us.

  4. Mark Martin

    This methodology is very resourceful. Nevertheless, once when I was at Lowell Observatory for a few days doing some research, I took time out to attend the public lecture on the history of discoveries made there. Included was, naturally, the map of Pluto’s surface features. When question & answer time came, I asked, “Has this work been reproduced, to verify the reliability of the map?” The lecturer hesitantly replied, “Uh- no. No, it has not.”

    I just couldn’t resist.

  5. blf


    To derail the topic slightly (sorry), I haven’t seen (apologies if I missed it) anything from The BA on this: Brightest ever supernova captured by Nasa telescope. This is seems rather amazing, albeit a completely different Vogan fleet. ūüėČ

  6. blf: Yeah, I can’t believe I wrote about that supernova before the “Big BA” did!! of course, he’s busier than I am.

    But on topic to this thread: When I was looking at all the “mapping” data, I thought that it would have been fun if – in places not really clear – it said “Here There Be Dragons.”

  7. DrFlimmer

    Supernova, of course, where is it? *gg* Thats more interesting then such exoplanets (do I make so enemys?? *gg*). And the supernova is maybe triggered by antimatter… We will discover a lot, golden age? maybe!

  8. Hey BA,

    I haven’t dropped a comment for a while, but dude I appreciate the fact that you explain things in layman’s (Rockstar’s) terms. Sometimes I get lost reading other science blogs, and I’m sure it’s because brilliant people have a hard time dumbing down to my level.

    Uh, was that a complement? I hope so…

  9. Remek

    The scientific pre-publication document became available just last night. I would imagine Dr. Phil would prefer to study the details for his post and commentary, rather than only relying on the PAO press releases and a press conference. :)

  10. Grand Lunar

    I’d first like to say I like blog entries like this better than the political ones. Not saying you should stop posting those, though!

    Anyway, this IS way cool! Mapping a planet 60 light years away! I can only wonder if they ever did something like this on “Star Trek”?
    Who knows what other wonders we’ll find?

  11. Gary Ansorge

    Planetary features from 60 Lt. Years away? How cool is that?

    Now all we need is a closer look at the earth like planet. What do we do if we find evidence of green stuff growing??? Transmit signals?

    Mr Spock, where are you?

    Gary 7

  12. icemith

    *Only* 60 light-years away! I’ve just realised that when light reflected off that planet, I was already 7 years old!

    Now that puts it in a personal perspective. Now I have to actually see this myself. Well maybe not the actual planet, but I believe I should be able to see the parent star. Please tell me I can, and where exactly, in the sky.


    PS. Happy 60th Birthday to the light that has just arrived today.

  13. tacitus

    There has certainly been lots of fascinating extrasolar planetary news in the past few days. The first results from the COROT mission is also cause for great excitement. Scientists studying the first returns now believe that COROT’s instruments may be 30 times more sensitive than originally specified, maybe even allowing them the chance of detecting near Earth-sized planets.

    JPL researchers also recently announced the successful laboratory test of a new type of coronagraph for the greatly improved blocking of a star’s light while imaging the planets in orbit around it.

    And I see that astronomers are holding out hope that something called “extreme adaptive optics” will keep ground-based observatories in the arms race for the hunt for and imaging of extrasolar planets.

    Interesting times indeed.

  14. Mark Martin


    Here’s some quick info on the star and where to find it in the sky:

  15. StevoR

    Astounding is too mild a word.

    **** We are seeing “weather” & atmospheric features on a planet orbiting under two days from it star sixty lightyears! ***

    Well done to all who achieved this because it really is an extraordinary breath-taking discovery! :-)

    We _are_ having a good run as someone above mentioned – to add to that list scientists discovered the most massive exoplanet yet known a week or so after Gliese 581c & d – this huge 8-X-Jove’s mass SuperJovian goes by the near-rhyming quasi-comical “name” of HAT-P-2b if y’all want to check that out too. lets just hope itcontinues!

    I for one am really looking forward to the first really earth-like planet discovery – a 2or so Earth-mass world orbiting a G-type star at about our distance .. ūüėČ

  16. StevoR

    Okay – inevitable correction coz its late & I’m apoor typer atthebest of times :

    ** We’re seeing “weather” & atmospheric features on a planet (_Or two_!) orbiting its star in under _three_ days from a distance of SIXTY light years away! ***

    Astounding, amazing, extraordinary … Nup need to say somethingstronger tiorefelct how remarkable that really is!

  17. And, according to a Scientific American article, the exoplanet’s temperature is a warm 1,200 kelvins on the day side and 970 kelvins on the night side (about 1,700 and 1,285 degrees F).

    When you go, don’t forget to pack your asbestos underwear. It’s toasty. And you should pack a good book for the trip (I recommend Bad Astronomy). At 150,000 km/h it’ll take you about 270,000 years to get there.

  18. Stu

    Yes, it’s a great achievement and all, and yes, the guys behind it are amazing, and in saying this I don’t mean to take anything away from their stunning achievement, but at the risk of sounding like a heretic here, I’m sorry, but I’m having a really hard time thinking of some of these rapidly-multiplying “exo-planets” actually as planets. I mean, come on… one of these “planets” whips around its sun in less than 3 days, and has a surface temperature of 2000deg c! That’s hotter than some low mass stars, isn’t it? The other one, that has a ‘year’ just 2 days long, has winds that reach a speed of 6000mph, or 30x faster than Earth’s jet stream winds.

    I’m getting very uncomfortable calling these exotic, truly alien objects ‘planets’, I really am. They’re more like a new class of dwarf star, or a “stunted star”, I reckon.

    Oh. Ok. Just me then. ūüėČ

  19. icemith

    Thanks again to Mark Martin for pointing me in the right direction. I looked-up the reference and see that it is quite close to M27, the Dumbbell Nebula. However I notice that it is given as 1360 light years away, so it is lucky that this little system was, from our perspective, not confused with, or even at some time in the past or maybe in the future, actually to be found super-imposed over that relatively distant nebula.

    How many other similar “pairings” have been found that in reality, are not related at all, but just happen to be in the line of sight to us? I realise that most of the traditional Zodiac Constellations are in this category, but those stars are actually very widely spaced in distance from us and each other.

    So I have to find M27 in my personal portable viewing apparatus, or better, but with our light-flooded city skies, I don’t hold out much hope. I’ll try anyhow.


  20. Buzz Parsec

    Stu –

    Despite their size, temperature, wind speeds, orbital speeds and other physical properties, these objects are much more closely related to the planets of our solar system than they are to stars. First and foremost, stars generate energy by nuclear fusion in their cores, or for the case of defunct stars (white dwarfs or neutron stars), they did do this at some time in the past. These planets are much too small to have ever generated the pressures and tempuratures required to fuse anything. Over 13 Jupiters is enough to collapse the core to the point where it resists further contraction due to electron degeneracy pressure. (See the Wikipedia article about Brown Dwarfs.) The exoplanets discovered so far are all too small (less than 8 M(J)) so are true planets.

    So a planet is a big round thing orbiting a star that is too small to be a brown dwarf but large enough that its own gravity has made it basically spherical. (Plus all that weird stuff about having cleared out its orbit… What about an object that is distinctly planetary in size but is only a few million years old and sitll has lots of junk in its vicinity? Like the Earth before it collided with the object that resulted in forming the Moon? Was there a time in our Solar system’s history when Jupiter, Mars, Venus and Mercury were all planets, but Earth, the 2nd largest among these, was not? And what of Saturn, Uranus and Neptune? Things happen slower out there, it’s colder and it takes much longer to orbit the Sun, so it probably takes longer to scatter or accrete all the raw material, so they may or may not have been “planets” yet… Can you tell I’m still bitter about Pluto getting stiffed? :-) :-)

  21. It looks from that press release as if they didn’t use the changes in brightness during eclipse by the star, they just measured brightness variations during planetary rotation to get the integrated brightness of longitudinal strips. (In fact, you can work out from the maps they must have done something like that, since the occultation method you describe can’t work for the anti-stellar hemisphere.)

    Since the limb of the star doesn’t line up exactly with lines of longitude on the planet as the one occults the other, it should theoretically be possible to use the occultation method to refine the map into something more truly two-dimensional for the sub-stellar hemisphere. Or maybe they did that but didn’t mention it.

    Buzz Parsec: of course it’s a planet, but not by the IAU definition which insists on the body being in our solar system.

  22. Mark Martin


    What matters isn’t how closely these objects resemble the familiar planets in our neighborhood. What matters is that they are being discovered, and measured. It allows for the spectrum of known possibilities to be broadened, and ultimately for our understanding of the kind of universe in which we find ourselves to be generalized and strengthened.

  23. Stu

    Thanks Buzz and Mark, appreciate the thoughtful feedback; I was expecting a lot of flak for my heretical views! I fully appreciate and understand that in all ways technical and official these objects ARE planets – being a children’s astronomy book author I know all the definitions and processes, as flawed as they are – but (and I actually think a lot of people feel this way too, but just don’t say it) there’s still a niggly voice at the back of my mind saying “Well, yes, it’s amazing we can do this, well done guys, but these things are so big and hot and behave so freakily that they’re not real planets, are they?” But it sounds like it’s just me… ūüėČ

    As for Pluto, well, that’s not over yet, I’m sure, and I made my thoughts on that plain here…

  24. Hi
    I have now read in a couple of places (think i have read…)
    that this planet completes an orbit in 2.2 days or so….
    isnt that a little fast, no matter how close it is to its star.
    Im no scientist, but im failry sure that there are still vast amounts of miles/km involved, and i dont see how or why a planet would orbit that fast?
    i mean, wow… it must have some speed!

    Please can someone explain this to me?


  25. Mark Martin

    Hi Chris,

    2.2 days does sound rather brief, but it’s not absurd. Let’s posit a satellite on orbit close to our sun. If we specify its orbital period t be just 2.2 days, the radius of that orbit comes out to a little over three million miles, a perfectly possible circumstance. It all just depends on the solar mass and the radius of the orbit.


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