What would sunset look like if you were on the planet HD209458b, a gas giant orbiting a star 150 light years away? According to exoplanetary scientist Frédéric Pont, it looks like this:
Isn’t that pretty? And there’s quite a bit of science in that, too.
First things first: HD209458 is a star pretty similar to our Sun. It was one of the first stars determined to have a planet orbiting it (way back in 1999) — the aforementioned HD209458b, nicknamed Osiris — and it turns out the planet’s orbit is so close to edge-on as seen from Earth that we see that planet passing directly in between us and that star once per orbit. When the planet transits that star the amount of light we see dips a little bit. From that we can get the period of the orbit and the size of the planet (a bigger planet blocks more light).
But we can get more, too. There’s a camera on board Hubble called the Space Telescope Imaging Spectrograph, or STIS. It can take the light from an object and break it up into thousands of separate narrowly sliced colors, called a spectrum. By analyzing that spectrum we can find out an astonishing amount of things about astronomical objects: their temperature, rotation, even their composition!
Shortly after HD209458b was discovered to be a transiting exoplanet, STIS was pointed at the star. The camera took hundreds of very short exposures during a transit in the hope of being able to detect the atmosphere of the planet. Osiris was known to be massive, about 70% as massive as Jupiter, so it most likely has a thick atmosphere. It also orbits so close to its parent star — 6.7 million km (4 million miles), much closer than Mercury orbits the Sun — that the heat from the star puffs the atmosphere up, making it easier to see.
In fact, the spectra did reveal the presence of an atmosphere; the first time the atmosphere of an alien planet was ever observed. Different elements and molecules absorb light at different colors, so in the spectrum there are dark spots where the planet’s air absorbs the light from the star behind it during a transit, and how dark that spot gets tells you how much light is absorbed.
It’s this information Prof. Pont used to create the image above (inspired by investigation and an animation done by Alain Lecavelier des Etangs). By knowing the color of the star itself, and using the way the planet’s atmosphere absorbs light, he created this image of the star using sophisticated computer modeling. The star itself is white, like the Sun, and so you might expect the sunset to look red like it does on Earth. But there are different processes involved with Osiris’s air! Sodium absorbs red light, and there’s enough of it floating around in the atmosphere of Osiris that the sunset takes on a bluish cast, but then as the star sets further, the blue light gets scattered away, much as it does here on Earth. The result is a green star — something not usually seen!
Pont also modeled the layering in the atmosphere too, and how each layer would affect the star’s color, producing the image seen. Even the glowing layers around the star are real (at least, real in the computer model); the reddish one is from those same sodium atoms re-emitting the red light they absorb, and the bluer layers from the light scattered away. By the way, he did this same analysis for the planet orbiting HD 189733, and got a far more terrestrial looking sunset.
Two things to note: the planet has no surface, so Pont put his imaginary sunset-watcher 10,000 km above the planet’s surface, observing as the star sinks beneath the planet’s limb. Imagine being in a space station (with the AC set to ultra-super-duper maximum), floating in front of a window, and seeing such a thing unfold! I would never have guessed the sunset would be green. Which brings me to the second thing: this model of the sunset is an average over the entire atmosphere. Where I live in Boulder, for example, the sunsets are different than they are in the eastern US, or in, say, Greenland, due to local conditions. What Pont did was take a planetary average for Osiris, since the STIS observations look at the whole planet all at once.
And a personal note, too. I was still working on STIS when these observations came in. Those were heady times; the idea of exoplanets was still pretty new, and being able to detect one this way was very new. I remember my boss, Don Lindler, very excitedly telling me he had the data from the observations and was going to do the basic processing of them for the scientists involved. He let me see them, and it was funny: to the eye, all the spectra (684 of them!) looked the same. But when you analyzed them carefully, subtle changes could be seen as the planet moved across the face of its sun. It was by far the best observations of a transiting planet ever seen. I remember Don and I were freaking out… well, I freaked more than Don did; I’m more of a dork. As a scientist working on Hubble I knew I couldn’t tell a soul about any of this — the investigators involved did the work, so they get the glory — and it was really tough. But oh, those few minutes of seeing that.
And at the time, of course, it didn’t occur to me this data could be used to model what a sunset would look like. I wish I had thought of that! But I’m glad someone else did, and made this dream-like vista. It’s nice to know — to see — the fruits of those earlier labors paying off so colorfully.
Image credits: Prof. Frédéric Pont at the University of Exeter; NASA/JPL-Caltech.
Related posts:
- A boiling superEarth joins the exoplanet roster
- Dry, hot, dusty alien worlds
- Video chat about the new Kepler planets
- More images of exoplanet show it orbiting its star
January 9th, 2012 at 7:27 am
I would love to see this in person, but suspect the properties of the system would be a bit harmful to direct observation.
Being this close to the star, what would the atmospheric temperature be and how significant would the solar winds affect be on the atmosphere in terms of bleeding it off into space. (would the other planets further out be seeing a large comet effect.
January 9th, 2012 at 7:33 am
Typo? “…the planet has no surface, so Pont put his imaginary sunset-watcher 10,000 km above the planet’s surface,…”
January 9th, 2012 at 7:55 am
Superluminous! (Beyond mere brilliance.)
That they can come up with this view backed with such good science is astounding. Thankyou BA & Frédéric Pont. I love your work here.
In a way I’m surprised the Osirian (spelling?) sun looks so small & wonder :
I) If you could make out some pretty clear prominces, flares and spots from this hypothetical space station or high altitude very hot air balloon / zeppelin vantage point?
II) If we get a “green flash” when our sun sets what would the equivalent be for teh last rays of HD 189733?
III) If this world is tidally locked -with maybe some libration effect – would this sun be a stationary object in Osirises sky – forever setting or rising but actually never doing either!?
@ 2. Bill3 – January 9th, 2012 at 7:33 am :
I’m guessing the last word there was meant to read “cloud-tops” instead!
January 9th, 2012 at 8:35 am
Incidentally, on a historic note, Osiris (HD209458b) was the very first extrasolar planet ever discovered / confirmed to exist using the transiting method and also the first to have its atmosphere detected – as it goes “boiling” off this exoplanets surface.
See : http://www.nasa.gov/vision/universe/newworlds/Osiris_leaks.html
where NASA notes :
Plus see : http://www.space.com/3673-water-extrasolar-planet-atmosphere.html
& of course :
http://en.wikipedia.org/wiki/HD209458b
I still find the discovery and topic of Hot Jupiters – these gas giant worlds as large or larger than Jupiter brushing right up against their alien suns so much closer even than Mercury to ours the most remarkable, mind-expanding and marvellous recent finds in astronomy. Love these new found worlds!
January 9th, 2012 at 8:58 am
@ 3 Messier Tidy Upper:
I never pondered on the ideea of a gas giant tidally locked with it’s star. And I quite can’t picture that – I’m thinking at the temperature gradient between the hot and dark sides that must generate hugenormously winds.
January 9th, 2012 at 9:31 am
I was wondering how we can tell the period of the stars orbit by the time it takes the planet to transit its star. How do we know the planet isn’t just crossing the edge of the star, causing a shorter transit, or how do we know that the super earths we find are not just short, partial transits from much bigger planets?
January 9th, 2012 at 10:18 am
Yeah “Osiris” is an easier handle than HD 209458b, but the usage of these nicknames seems to be dying out somewhat. The only other one that still seems to get occasional usage is “Bellerophon” for 51 Peg b, and the less said about the usage of “Goldilocks” for 70 Vir b the better!
@5: the bulk rotation of the planet should be tidally locked, but the weather on such worlds is probably extreme. Most models suggest that hot Jupiters should feature extremely fast equatorial jets and large polar vortices: the circulation patterns would not look anything like Jupiter. This hasn’t stopped various space artists from continuing to depict them as Jupiter-like, multi-banded planets though… oh well.
January 9th, 2012 at 10:19 am
Im so glad I live on earth lol. We have varieties of gorgeous ones. lol
January 9th, 2012 at 10:23 am
I would think a gas giant that close to its parent star would have been obliterated like a puffball in a blast-furnace. Another physics kick to the noggin.
January 9th, 2012 at 11:25 am
A planet that close to its star might have epic arorae, and maybe a tail of material being blown off the upper atmosphere slowly by solar wind. I wouldn’t expect it to combust like a puffball, since you’d need a reacting fuel like oxygen to ignite the hydrogen and methane.
January 9th, 2012 at 11:36 am
Sometimes I think science should be considered an art form, especially when scientists can produce such beautiful images.
January 9th, 2012 at 12:07 pm
What would a similar image from a similar location in the atmosphere of Jupiter, or Saturn, look at?
January 9th, 2012 at 12:17 pm
@3. Messier Tidy Upper
“If you could make out some pretty clear prominces, flares and spots from this hypothetical space station or high altitude very hot air balloon / zeppelin vantage point?”
AFAIK we could probably see prominences from Earth quite easily, without telescopes even, if not for the Sun itself. This isn’t a function of distance so much as relative brightness, like a candle lit by a forest fire behind it. Don’t we have to look at the Sun in wavelengths it’s relatively weak in, like UV, to see these features?
@7. andy
At some point it became clear we were discovering exoplanets much faster than we could find names for them. We kind of stopped with KBOs as well; there’s too many of them.
@9. Robin Byron
Not a kick to the noggin, really. Osiris is probably losing mass at a very high rate (and may eventually become a scorched naked ball like Mercury) for precisely the reason you’re envisioning. However, its mass is comparable to Jupiter, so what you’re really doing is drastically underestimating Jupiter’s mass. Here’s some perspective: Unlike other planets, the center of mass between the Sun and Jupiter lies outside the Sun’s surface. Jupiter doesn’t orbit the Sun so much as the two orbit a common point in space. The Sun is much more massive, no doubt, but Jupiter is MASSIVE. It’s so big its gravity tortures its inner moons and has moons orbiting as far as 30 million km from it.
So, the biggest problem in your basic analogy is the puffball. This is more like a 40-foot-high mountain of snow thawing in sunlight; the last one I saw like that took until summer. The star can’t lose, but there’s just too much stuff to get rid of all at once. More stuff also means a slower start. Like how the outer layers of the snow pile insulate the inside until it’s dissolved away, a higher starting mass means a bigger gravity well for the star to blast material out of. It’ll happen, but it’s gonna take a while.
January 9th, 2012 at 12:28 pm
@ 7 Noel:
Well, yes, but… from what I know, the bulk of the planet doesn’t have a uniform rotation speed – higher at the equator, slower at the pole and it’s like this because there is much less friction between inner layers in a gaseous planet than in rocky one, so different layers can move at different speeds. And since there are no fixed surface features, I don’t know how you could consider the planet tidally locked except by comparing its rotation period with its orbital period. But what rotation period: mean, equatorial, polar? What about convection in the core
Like I said, it’s difficult for me to imagine a hot Jupiter tidally locked – I guess it should resemble a laminar flow, but this is not possible as the fluid speed at the poles must be lower than at the equator (smaller circumference -> less distance to travel in the same time interval) so you must have at least several chaotic decoupling zones between different flow speed regions.
January 9th, 2012 at 12:35 pm
I’m wondering what a tidally-locked gas giant would look like from the distance of a Cassini or Galileo probe. I’d assume there wouldn’t be bands of different gases like we see on Jupiter and Saturn, but there might be some interesting patterns produced by convection currents in the gas as it circulates from the terminator to the superheated center of the star-facing side. So it’d look like a sort of glowing eyeball.
And how does tidal locking work with a gas giant anyhow? If there’s no solid surface, what is it that actually locks?
January 9th, 2012 at 12:51 pm
“the planet has no surface, so Pont put his imaginary sunset-watcher 10,000 km above the planet’s surface”
Huh? I’m confused now. Should that read 10,000 km above the planet’s center of mass?
-kap
Edit: Should have read the comments, seems Bill3 already pointed this out.
January 9th, 2012 at 1:14 pm
@15. The Math Skeptic
Don’t mean to go all “Zen” on you, but a gas giant can be tidally locked in the way air can have a “stationary front”. If enough molecules are moving in the same direction — on average — then you can measure the wind speed/angular momentum. If that relative speed is zero, then the air is “stationary”.
As for the pattern, I’d imagine this planet would have granulation like the Sun’s mixed with swirls caused by hot air mixing with cold. It’d probably be quite colorful, and star would be constantly blasting off a thin haze of hydrogen. The question is what features would dominate the surface — er, “cloud tops”, and how big they’d be.
January 9th, 2012 at 1:17 pm
It would be really neat to see a sunset like this, but if there are particulates in the atmosphere, they’d scatter short wavelengths much more than long ones, resulting in a very dim red or perhaps colorless sunset (since sodium absorbs the red and the particles scatter the blue). Maybe the sun would disappear before setting like it does with very thick haze on earth.
January 9th, 2012 at 2:59 pm
I’d be interested to see what the computer model would render using similar data from our own planet!
January 9th, 2012 at 3:58 pm
Dang!! That must have been so exciting, Phil! Taking an atmospheric spectrum from an exoplanet still blows my mind even after having heard about it years ago; I can barely imagine being there and getting that data for the very first time!
As for the pattern, I’d imagine this planet would have granulation like the Sun’s mixed with swirls caused by hot air mixing with cold. It’d probably be quite colorful, and star would be constantly blasting off a thin haze of hydrogen. The question is what features would dominate the surface — er, “cloud tops”, and how big they’d be.
I wonder about the granulation too. In our sun, I thought it was a result of the sun radiating heat more or less evenly from deeper layers, so convection cells there tend to be small and uniform? “Small” being relative, of course
Anyway, that’s a fascinating point that you bring up! Even with 4 nicely varied gas giants in our solar system to photograph, they’re all pretty darn far from the Sun. We’ve never taken a close-up of a “hot Jupiter” and have nothing that’s anything like one for comparison. As extreme as weather on Jupiter is (by our standards), I can only imagine that near a star, there must be phenomena that we’ve just never seen! I’m picturing storms that dwarf the great red spot, or glowing incandescent jet streams streaking around the dark limb…
On another note, I think we Slashdotted the server.
January 9th, 2012 at 4:03 pm
I was wondering about the sodium layer in the Earth’s atmosphere, so I did me some Googlin’ and found that it’s thought to come from meteors.
Would that indicate that this planet gets hit by a fair number of objects, even as close to its primary as it is? Or is it just so darn hot that the sodium that was there when the planet formed gets lofted into the upper atmosphere?
January 9th, 2012 at 4:14 pm
@6 Noel: I was wondering how we can tell the period of the stars orbit by the time it takes the planet to transit its star. How do we know the planet isn’t just crossing the edge of the star, causing a shorter transit, or how do we know that the super earths we find are not just short, partial transits from much bigger planets?
I could be wrong, but I think they can look at several characteristics of the numbers and nail things down somewhat from those. For instance, if you see multiple transits, you can nail down the distance and speed of the planet’s orbit, and the star’s size can be deduced from its spectra, temperature, and brightness. From that you can expect a certain range of transit times. Also, for a planet just “brushing” the edge of the disc, the light curve will be far “smoother” then one where the planet passes across the edge of the star’s disc where it’s almost perpendicular to the planet’s motion. Think of running into a circle head-on versus a glancing blow. If I’m making any sense…
January 9th, 2012 at 4:38 pm
@The Math Skeptic (#15):
“And how does tidal locking work with a gas giant anyhow? If there’s no solid surface, what is it that actually locks?”
One might just as well ask: If Jupiter has no solid surface, then what does its quoted period of rotation refer to? (The answer: the period of rotation of its magnetic field, which is presumably tied to its less fluid innards.)
Thus, a tidally locked “gas” giant (I hate that term! Jupiter, Saturn, Uranus, and Neptune are mostly _liquid_, not gas) would presumably have a core whose spin would be locked to its orbit in a 1:1 resonance. It’s not clear to me that, in that situation, the atmosphere would experience any sort of globally organized motion. But perhaps there are planetary scientists among us who could expand on that thought?
January 9th, 2012 at 4:41 pm
With that much thermal input, massive currents are essentially impossible to NOT form. With their motion, deeper currents form. One wonders then how intense the magnetic field is for the planet and what impact on atmosphere loss a strong magnetic field may have.
Of course, inverse square will have to be considered, as the atmosphere is expanded away from the dynamo’s complex fields.
Anyone have supercomputer time and a good coder?
January 9th, 2012 at 5:12 pm
“the planet has no surface, so Pont put his imaginary sunset-watcher 10,000 km above the planet’s surface”
Osiris is a gas giant. It has no physical surface, it’s a gas giant. You wouldn’t be able to physically “stand” on the surface, because our mass and the gravitational pull of the giant would just suck us into the center of the gaseous planet.
So the surface of the planet is actually gas, when Pont put his imaginary sunset watcher above the planet’s surface, he used the edge of the gaseous state as his “surface”.
January 9th, 2012 at 7:05 pm
The alien sun is surely blasting that jupiter class planet ‘apart’, but I wonder if a CME form said sun would potentially add mass to the planet (probably not as fast as it leaves)… but then maybe if enough mass from the star (or alien sun) goes to the alien gas giant – it could – maybe – in my medicine is correct – lead to a binary star system, but then its only 70% the mass of Jupiter … and i think you need a 10x mass of Jupiter to get a small star..
Of course we could measure it in 50 years and see if its mass has gone up or down significantly – that would be interesting – assuming we have a civilization in 50 years.
January 9th, 2012 at 7:09 pm
@25 drx1: IIRC, a coronal mass ejection is almost all hydrogen, which also tends to be the easiest gas for solar wind to strip away, so I can’t really imagine the planet gaining mass from the star faster than it loses it.
That would be very cool if our timing was sensitive enough to register a mass gain or loss. I’m not sure what kind of numbers we’d be talking about one way or the other.
January 9th, 2012 at 10:43 pm
Doctor, SOME modes call for a metal core, some not. THEN, *SOME* call for a hydrogen metal core, others carbon. Lacking further information, one cannot decide upon ANY, as you have chosen.
Unless YOU have facts otherwise not in evidence here or the originating article.
January 9th, 2012 at 11:35 pm
@ ^ Wzrd1 : January 9th, 2012 at 10:43 pm : Yep. We still know so little really about what
the cores of our own systems gas giants are like let alone other alien planets. What lies at the centre of Jupiter remains largely a mystery as I understand it.
@13. Dragonchild :
Nice analogy and explanation there.
From my understanding – possibly mistaken – some very near-to-their-suns “superEarths” (Misleading term if ever there wa sone given how unearthly such worlds would be!) such as Corot Exo-7b may just be the remnants of former Hot Jove Pegasids or “Roasters” as they’ve also been termed.
One study “How to Destroy a Giant Planet” on space-dot-com by Aylward, Koskinen & Miller back in Dec. 2007 suggests that all HotJoves closer than 0.15 AU (24 million km) will eventually be destroyed by this catastrophic evaporation. (Including 51 Pegasi b, Tau Bootis b, HD 209458 b & more.)
Good point. I guess that would be right. Our Sun is magnitude – 27 or so from Earth. From a Hot Jupiter it’s apparent magnitude would be, well astronomical I imagine!
@19. Steve : January 9th, 2012 at 2:59 pm
I can’t say for sure – only the author here could – but I would expect they tried exactly that as a control test to make sure the program / simulation systenm as working properly.
January 9th, 2012 at 11:55 pm
See :
http://www.space.com/4705-destroy-giant-planet.html
for an article on that study by by Aylward, Koskinen & Miller on how close a roaster Hot Jove can get.
See :
http://en.wikipedia.org/wiki/COROT-Exo-7b
for the planetary facts sheet on Corot-exo-7b.
Plus see :
http://www.csmonitor.com/Science/2010/1013/Gliese-581g-Goldilocks-planet-might-not-exist-after-all
For the person here who mentioned the Goldilocks planet.
January 9th, 2012 at 11:58 pm
@11. James Pailly : “Sometimes I think science should be considered an art form, especially when scientists can produce such beautiful images.”
Seconded by me. There’s a science to art and an art to science I think! Combining them like this is a wonderful thing and wish there was more of it.
12. Tara Li : “What would a similar image from a similar location in the atmosphere of Jupiter, or Saturn, look at?”
A much smaller sun! From Jupiter or Saturn the Sun would be close to star-like I think witha very small diameter and, I presume perhaps incorrectly that it would white or yellow in colour in colour until it quickly blinked out. Some of our spaceprobes have almost been close enough to witness a Jovian / Saturnean sunset firsthand.
January 10th, 2012 at 1:11 am
@7. andy :
Well, the “Goldilocks Planet” tag as I’ve heard it applies aptly to Gliese 581 g & suits that pretty well – if that one actually exists.
I much prefer names to catalogue designations as more memorable, appropriate and evocative. I know there’s a lot of exoplanets but I wish at least some nicknames would be made official and used more frequently.
Exoplanets with nicknames include :
1. Bellerophon for 51 Pegasi b.
2. Osiris for HD 209458 b.
3. Polydeuces for Pollux b the planet around the brightest exoplanet-hosting star so far.
4. The Genesis / Methuselah Planet for PSR B1620-26 b -the oldest known planet located in M4.
5. Tatooine for HD 188753 b the planet discovered in a triple star system.
6. Hoth or OGLE-05-390 L b the furthest exoplanet ever found – via micolensing 21,000 ly off.
7. The Balsawood Planet or TrES-4 which has the largest diameter and least density; 70 % larger than Jupiter’s radius but a density equivalent to balsawood.
8. The Hot Ice Planet or Gliese 436, the first of its planetary class to be discovered. Too small to be a gas giant, it is almost certainly made from high pressure and temperature exotic ices.
9. The (ex?-)Goldilocks Planet or 70 Virginis b which could also be a brown dwarf companion and is in an elliptical orbit coming as near its star as Mercury gets to our star.
&
10. The (maybe?) Goldilocks Planet or Gliese 581 g which is a “SuperEarth” in the Habitable Zone of its star. If it exists at all which unfortunately may well not be the case.
Plus recently Kepler 22b has been variously dubbed “Earth 2.0 ” and “SuperEarth” by some of the media which are probably once again being a little inaccurate and premature in their judgement. Sadly, the hype over this world (like that over Gliese 581 c before it) probably isn’t justified and it is most likely another gas dwarf / Hot Neptune type world.
January 10th, 2012 at 2:08 am
The odd letters and numbers are indeed a mouthful, but we’re definitely going to run out of mythological names sooner rather then later. How about we name planets based on certain characteristics (e.g., that Tatooine planet) and then a simple number (Tatooine 1, 2, 3, etc)? These wouldn’t be the scientific designations, of course, just easier handles such as Osiris. That way astronomers won’t have to spend all their time going through old anthropology textbooks
January 10th, 2012 at 12:54 pm
[...] we can’t imagine it with great accuracy. Equipped with careful observations, it’s possible to visualize the sunset as it would look from a distant exoplanet. This is what it looks [...]
January 11th, 2012 at 11:06 am
The “Exo” part of the CoRoT planet designations is sooo last decade… they removed it in March 2009. It’s just plain CoRoT-7b now…
January 12th, 2012 at 8:40 am
[...] As Osiris orbits so close to its star, atmospheric temperatures are high (around 1,000 degrees Celsius or 1,800 degrees Fahrenheit). this means its outer atmospheric layers become “puffed up.“ [...]