Back in September I posted an image taken from the Curiosity rover showing Phobos, one of the moons of Mars, crossing the face of the Sun.
That was pretty cool. But this is cooler: video of Phobos transiting the face of the Sun seen from the rover Opportunity on September 20, 2012!
Lest it be overshadowed (HAHAHAHA! Get it?) by Curiosity, remember Opportunity is still going strong after more than eight years on the surface of the Red Planet. These shots from the elder rover are really awesome; Phobos is not even close to being a sphere and you can see its potatoey lumpiness in the animation.
Phobos is about 27 km (17 miles) across its long axis, which is small for a moon. It looks big because it orbits Mars so close in; it’s only 6000 km above the surface. It was actually a bit farther away from Opportunity when these images were taken, making it look smaller than it could be.
In fact, given its size and distance, Phobos has a maximum size in the sky of about a quarter degree, or half the size of our full Moon. As seen from Earth, the Sun and the Moon are about the same size in the sky. But Mars is farther from the Sun, so the Sun looks smaller, about 1/3 of a degree. So even at best Phobos can’t completely block the Sun.
But… Phobos isn’t in a stable orbit. Tides from Mars are dropping it down closer to the planet, making it appear bigger. In a few million years it’ll drop low enough to create total eclipses as seen from the surface of Mars. They won’t last long, since the moon is zipping along pretty rapidly in its orbit. Still though, I have to admit to a bit of delight: creationists like to claim the Earth is special, and we’re the only planet that has the right conditions for total solar eclipses. That’s not even really true right now, and it certainly won’t be once Phobos dips down a bit more.
Of course, once Phobos gets too close to Mars a few million years later it’ll crash into the surface, making the sweatiest apocalyptic scenarios dreamed up by humans look like a warm summer’s breeze by comparison. Nature! It has a way of making our fevered imaginations look like pretty small potatoes.
Image credit: NASA/JPL-Caltech/Cornell Univ./Arizona State Univ. Tip o’ the rocket crane to Mars Curiosity on Twitter.
OK, this is simply too cool.
The Mars Curiosity rover has already returned thousands of images taken of the Red Planet’s landscape. But on September 13, 2012, it was commanded not to look around, but to look up, at the Sun. Why? Because Mars’s tiny moon Phobos passed directly in front of the Sun, partially eclipsing it!
Sweeeeet. I blew the original image up by a factor of two for clarity.
Technically, this is called a transit – when a much smaller body passes in front of a larger one. Usually, there’s some science that can come from this; the timing of the transit gives a better orbit for the moon (since the rover’s location on the surface is precisely known), and so on. In this case, though, we study Phobos with other orbiting spacecraft, so I’d think its orbit and position are extremely well determined.
It may very well be that this shot was taken just because it’s cool. I actually kinda hope so.
It’s not the first time a Phobos transit has been seen; in fact it’s been done several times. Here’s a video of one seen by the rover Opportunity in November 2010:
Wikipedia has more info. I’ll note that as of right now, the image above is the only one I’ve seen listed on the Curiosity raw images page (at decent resolution, that is; there are lots of tiny thumbnails, and bigger, cleaner versions should show up soon). The image was taken by the MASTCAM, which has a filter on it so it can observe the Sun. It does that for various reasons, including being able to observe how much the Martian atmosphere is absorbing sunlight.
Phobos orbits Mars pretty close in, just about 6000 km (3600 miles) above the surface of Mars – compare that to the 400,000 km distance from the Earth to the Moon! Phobos is so close that it transits the Sun pretty much every day for some location on Mars, making this something of a less-than-rare event. It’ll only be a year before it happens again at Curiosity’s location.
Still. It’s an eclipse, seen from Mars, taken by a nuclear powered one-ton mobile chem lab that we put there. I think that qualifies as pretty damn cool.
Image credit: NASA/JPL-Caltech/Malin Space Science Systems. Tip o’ the heat shield to… MarsCuriosity on Twitter!
- Curiosity’s self-portrait
- Curiosity looks Sharp
- Curiosity rolls!
- Now you will feel the firepower of a fully armed and operational Mars rover
- Gallery – Curiosity’s triumphant first week on Mars
Since the first planet was discovered orbiting another Sun-like star in 1995, nearly 800 more have been discovered. Only a handful have been directly detected: most are discovered by their influence on their star, either by tugging it or blocking its light as the planet orbits (at the bottom of this post is a gallery of images of exoplanets detected in these ways). But some have been directly seen: either glowing by their own light, reflecting that of their star, or — ironically — seen when they’re not seen.
Say what? OK, this takes a sec to explain, but it’s cool.
The star 55 Cancri hosts at least 5 planets. Located 40 light years away, it’s one of the closer planetary systems, and has been intensely studied. One of the planets, 55 Cancri e, is bizarre: it’s twice the diameter of the Earth and has 8 times our mass. It’s thought to have a dense core surrounded by water… but Earth-like it ain’t. It orbits its star in a very tight orbit, circling it once every 18 hours. It’s so close to the star that the surface temperature is probably around 1700°C — or 3100°F! That’s hot enough to melt lead.
So yikes. If it does have water, it’s in the form of a weird super-heated steam only held to the planet due to its strong gravity. Even then, the atmosphere may be boiling away like a gigantic comet. So again, this isn’t like Earth at all. Even Venus isn’t this unpleasant, and on Venus it rains sulfuric acid.
Anyway, an object at that temperature will glow in the infrared, quite strongly. If it were sitting all by itself in space, it would be easy to see. However, it’s sitting next to a star which is millions of times brighter, making it a significantly more difficult target.
… but not impossible. Read More
I’m very excited about all the news we’re getting of planets orbiting other stars. For Q&BA I got a good question about them: How many exoplanets are there?
[Note: the aspect ratio on this video is messed up a bit, like it was on the last one. I understand the problem now, but cannot fix it in this video. They should be back to normal next time!]
I wonder how many of the thousands of candidate planets known will turn out to be real? Probably most of them. And there are billion, hundreds of billions, of planets in our galaxy alone! How many of those are like Earth? Maybe soon we’ll know.
I don’t care if it’s a curse or not. We do live in interesting times.
I have an archive of Q&BA links and videos. Take a look and see if there are other ones that tickle your imagination.
- Q&BA: Which moon has the best chance for life?
- Q&BA: Can we build a space habitat?
- Q&BA: The Science of Science Fiction
- Q&BA: How does a gravity slingshot work?
- Q&BA: Why spend money on NASA?
- Q&BA: What happens if you are exposed to the vacuum of space?
If you’ve been outside after sunset the past few weeks, there’s not much chance you’ve missed Venus shining like a laser in the west. It’s obvious enough anyway, but the conjunction (close pass) of Jupiter really made this a sight to see!
And as lovely as it is to look at with the eye, Venus is starting to get interesting through a telescope now as well. Venus has phases, just like Moon. It orbits closer to the Sun than we do, so sometimes we see it on the far side of the Sun, and sometimes it’s between us and the Sun. When it’s on the other side of the Sun we see it fully illuminated, and as it gets between us and the Sun it appears as an ever-thinner crescent. Hopefully the diagram here will help (click to embiggen).
Right now, Venus is just "rounding the corner" of its orbit; the past few weeks it’s been heading away from the Sun from our viewpoint, and very soon will reach its greatest elongation in the sky from the Sun. At that point, every day will see Venus get a bit closer. Right now, Venus is very close to being half full.
"Amateur" astronomer Emil Kraaikamp observed Venus on March 15, and took this very nice shot of it:
Venus is shrouded in clouds, making it relatively featureless when you look at it through a telescope. However, if you use a filter that lets in ultraviolet light, some faint and subtle features in the clouds can be seen. Emil’s picture did just that, using a UV filter plus one each of red, green, and blue to get a true color plus UV picture. The phase of the planet is obvious enough, and you can also spot some of the patterns to the clouds, too.
Another astrophotographer, Alan Friedman, also took a stab at Venus (this time on March 17) and got the picture shown inset here. Again, you can see some detail, but clearly it’s not easy to get the goddess of love to reveal her secrets!
Venus reaches its maximum distance from the Sun in our skies on March 27. After that it starts moving closer to the Sun. It’s not physically getting closer to the Sun, it’s just moving between us and the Sun, but to us when we look at it in the sky we’ll see the two getting closer together.
But in physical fact Venus is getting closer to Earth, so even as it becomes a thinner crescent it will be getting bigger and bigger as our distance narrows, making it a great target even for binoculars. This means it will be getting even brighter in our sky! So if you think it looks like a beacon glowing to the west now, just wait a few weeks.
… and then the most amazing thing will happen: Venus will pass directly across the face of the Sun! This is an incredibly rare event. These transits, as they’re called, occur in pair separated by 8 years, but each pair is separated by more than a century. The last one was in 2004, but the next won’t be until December of 2117!
I’ll have more info about this event soon. The transit happens on June 5/6 2012, so you might want to make space in your calendar. Odds are pretty good this will be your only chance to ever see it again.
You would think that, of all the astronomical measurements we could make, one of the best known would be the diameter of the Sun.
You’d be wrong. It’s actually really hard to measure! For one thing, we sit at the bottom of an ocean of air, gas that is constantly moving around, mucking up precise measurements. For another, our telescopes themselves can be difficult to calibrate to the needed accuracy to get a really solid measurement of the size of the Sun.
Nature, however, provides us with a way to measure our nearest star. Naturally! And it involves Mercury, the smallest and, critically, the closest planet to the Sun.
In 2003, and again in 2006, Mercury passed directly across the face of the Sun as seen from Earth (the picture above is a view of the 2006 transit as seen by SOHO; a very short animation was made from this as well). Mercury’s orbit is tilted a little bit with respect to Earth’s, so these transits don’t happen terribly often, occurring only every few years. But because we know the orbit of Mercury so well, and our own distance from the Sun, by precisely timing how long it takes the diminutive world to cross the Sun, we can get a very accurate measurement of the Sun’s diameter.
A team of scientists did exactly this, using SOHO, which is a solar observing and solar-orbiting satellite. Because it’s in space, it doesn’t suffer from the problems of peering through a murky, dancing atmosphere. They were able to measure the timing of Mercury’s passage of the Sun to an accuracy of 3 seconds in 2003 and 1 second in 2006. They had to take into account a large number of effects (the Sun’s limb is darker than the center, which affects timing; they had to accurately measure the position of Mercury; they had to account for problems internal to SOHO like focus and the way it changes across the detector; and, of course, correct for the fact that Mercury cut a chord across the Sun and didn’t go straight across the diameter — but that only took knowledge of Mercury’s orbit and some trig) but when they did, they got the most accurate measure of the Sun’s diameter ever made: 1,392,684 +/- 65 km, or 865,374 +/- 40 miles.
That uncertainty of 65 km is quite a it better than what can be done from the ground, amazingly. It may sound like a lot, but it actually represents an accuracy of 99.995%! The Sun is big. Really, really big.
… and they’re not done. The authors are going to observe the Transit of Venus coming up in June, hoping it’ll improve their measurements. I’ll be very curious to see how that goes; Venus has an atmosphere which I would think would confound the observations. They may have ways around that though.
Either way, I think this is completely fascinating. Even thrilling! The Sun is the brightest thing in the sky, the center of our solar system, the basis of light and heat and life on Earth, and the best-studied star in the Universe.
And here we are, just now figuring out how big it is. Sometimes the simplest things can be the hardest, I suppose.
Image credit: NASA/EDA/SOHO
[I'm trying to catch up with all the news that's been released this week while I was off lecturing in Texas. This is Part 2 of a few articles just about exoplanets. Part 1 was posted earlier.]
Astronomers have found one of the most interesting exoplanets yet: one with a very extended ring system!
[That's an artist's impression of the system; click to encronosate.]
The planet was discovered with the SuperWASP (Wide Angle Search for Planets) telescopes — a UK project that employs low-magnification but very sensitive cameras which can observe large areas of the sky at the same time. It orbits a young star called 1SWASP J140747.93-394542.6, which is 420 light years away. The star’s youth — 16 million years — indicates that the rings are probably the leftover remnants from when the planet formed.
The planet and its rings were discovered using the transit method: looking for small dips in starlight as a planet passes directly between us and the star. This is how the vast majority of exoplanets are found. Usually, when you graph the brightness of the star over time, the dip in the plot as the planet transits the star starts suddenly, drops to some minimum, then jumps back up (see here for example). The whole thing is usually over in a matter of hours at most.
But this planet took nearly two months to transit the star! And the dip was weird: there were multiple times the star dimmed then got brighter again, at one point having 95% of its light blocked. Even though the planet wasn’t seen directly, the most obvious explanation is a ring system similar to Saturn’s (though much larger), blocking the light. It must have gaps in the rings, like Saturn’s do, to explain the starlight jumping up again over time. Overall, four rings were detected, and they stretch tens of millions kilometers in diameter!
Saturn’s ring are only about 300,000 km across, so clearly this planet must be much more massive than Saturn, and the rings denser. It may be a little unfair to compare it to Saturn at all; it’s more like a super-Jupiter still surrounded by primordial debris. Unfortunately, we don’t know how massive the planet itself is; you need Doppler data for that and none has been taken yet. The astronomers who discovered this system, of course, are looking into obtaining Doppler data. It’s even possible the object is so large it’s actually a brown dwarf and not a planet.
Perhaps most intriguing about all this are those gaps in the rings. The easiest way to explain them is that there are objects there, moons, sweeping out the material in the rings. Saturn’s rings have gaps for this reason. In fact, there are hundreds of gaps in Saturn’s rings! These are caused by resonances: if a ring particle orbits twice for every one time a moon orbits, for example, the moon’s gravity tugs on it every time it swings by, pulling it into a different orbit. Over time, all the particles in that orbit are gone, leaving behind a gap.
If the planet itself is big, how big are those moons? Could one be Earth-sized? It’s an idea that’s been around awhile, but none has ever been seen… yet. All these super-Jupiters being found have a lot of gravity, and it’s possible they have big moons. We’re also getting better at detecting smaller objects, so it wouldn’t surprise me if that announcement is made sometime relatively soon, too!
I’ll note that the idea of looking for rings and moons is more than an idea: the Hubble observations of the star HD 209458 I mentioned the other day were taken to look for moons and rings around that planet! None were seen, but astronomers will keep trying. There are a lot of planets out there, and one thing we’ve learned is that variety is the spice of nature.
Image credit: Michael Osadciw/University of Rochester
There’s been so much exoplanet news this week! I was in Texas the past couple of days giving a bunch of talks, so I’m trying to catch up. All the exoplanet news is way cool, but too much for one post, so I’ve split them up. I’ll post the other parts shortly.
Part 1: A trio of hot little rocks
First up? The three smallest exoplanets found so far. I usually don’t like to write about incremental discoveries, but this one is truly cool: all three orbit the same star, and all three are smaller than Earth! Any one of these would be a record breaker, but to find all three at once, in the same place? Amazing.
They orbit the star KOI-961 (short for Kepler Object of Interest), and were observed by the Kepler Observatory (details on how that all works can be found here). They all orbit the star extremely close in: the farthest one is a mere 2.3 million km (1.5 million miles) from the star! They’re so close they all take less than two days to circle it once. And even though the star is a red dwarf, and therefore relatively cool, they are so close to it that they probably resemble airless, heat-blasted Mercury more than Earth. They are almost certainly rocky/metallic bodies, since they are so small: 0.78, 0.73 and 0.57 times the diameter of the Earth. Although we’ve been surprised before, it’s hard to imagine anything that small could hold onto much atmosphere when they are so hot.
Funny, too: the star is tiny, only a bit bigger than Jupiter. And the planets are so close in the KOI-961 system looks more like Jupiter and its moons than our own solar system! The artwork above drives that point home. Everything there is to scale: the relative size of the star, the planets, Jupiter, and its moons. [Edited to add: Note that the distances are not to scale!]
Why is this news important? Well first, it adds more weight to the idea that planets smaller than Earth exist and can be found around other stars. Second, it shows that red dwarf stars can form and hold onto planets… which itself is important because red dwarfs are by far the most common kind of star in the Universe. They make up roughly 80% of the total number of stars! So finding multiple planets around one means, once again, planets are almost certainly common in the galaxy.
And third, it just shows once again that the Universe is a surprising place. This mini-solar system proves that nature is diverse, and will fill any niche it can. It also implies, very strongly, that we need to broaden our concepts of how solar systems form, what they look like, and how they behave.
Image credit: NASA/JPL-Caltech
- Kepler finds a mini solar system!
- Another Kepler milestone: Astronomers find two Earth-sized planets orbiting the same star!
- No, it’s *not* the smallest exoplanet found!
- A boiling superEarth joins the exoplanet roster
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. Read More
[NOTE: I have been informed that this is NOT the first planet seen in the habitable zone of another star, but the first seen by Kepler, and moreover the first that is not a gas giant. Rather than try to correct the text below using strikethroughs, which would be confusing, I simply edited the text. I hope that's clear!]
This is pretty big news: the space-based Kepler observatory has confirmed it has found its first planet in the habitable zone of a star like the Sun! Not only that, the planet may well be similar to Earth, though that’s not clear yet.
The planet, called Kepler-22b, is about 600 light years away. The star it orbits, called simply Kepler-22, is a bit lower mass and cooler than the Sun. The planet takes about 290 days to circle the star once, and as soon as I saw that number I let out a little "yip" of surprise — that number’s perfect! Why?
Because that puts the planet inside of that star’s habitable zone, the distance where, given certain planetary conditions, liquid water can exist. It may be that life can arise where there’s no water, but we know life on Earth needs water, so if we’re looking for habitable planets it makes sense to look for the possibility of water there.
The planet is closer to its star than Earth is to the Sun — that’s why its year is shorter — but the star is cooler, compensating for that. That makes this the best candidate yet for Earth-like conditions. But is the planet like our own world?
That’s hard to say.