The entire universe in blog form

July 1 2010 7:00 AM

The best planet pictures in the solar system

By Phil Plait

Bad Astronomy Gallery

(click any image to see it full size)

My Favorite Planet Pictures

To celebrate the Hive Overmind's Discover Magazine's new picture gallery software, I've collected my favorite pictures of all the planets in our solar system and put them together here for your viewing awesomeness. I've also thrown in the Sun and Moon, as well as one bonus surprise at the end. Each picture has a brief description, a link to the original higher-res version, and also a link to a blog post I've written with more information.

My picture choices may surprise you. I didn't just pick them for their beauty, but also for the story they tell, what happened behind the scenes, and simply because they're cool. I hope you agree. Whether you do or don't, leave a comment and link to your faves!

Solar system picture credit: Wikipedia/NASA

The Sun

There are literally millions of pictures of our nearest star, ranging from images of it as a plain, spotless disk to incredible close-ups of the roiling, churning surface and explosive flares. But there's something about this image that really grabs you!

Look to the upper right: see those shadows? Those aren't sunspots: they're the International Space Station and Space Shuttle Atlantis silhouetted in front of the Sun! This image was taken by the incredible "amateur" astronomer Thierry Legault in May 2010. Because the Sun is so bright, the exposure time is very short, freezing out the usual atmospheric blurring. That makes the picture extremely crisp and details easy to spot -- see for yourself in the super-high-res version. And don't be fooled by the apparent motionlesness of the duo: screaming above the surface of the Earth at 8 km/sec (5 miles/sec), they transit the Sun's face in less than a second! It took a lot of planning and good timing to pull off this amazing picture.

Related post: ISS Shuttle transit the Sun

Credit: Thierry Legault

Mercury

Even through the best telescopes on Earth, the closest planet to the Sun is a bit blurry. Because it never strays far from our central star, Mercury is always low to the horizon at twilight and difficult to observe. That's why NASA sent the MESSENGER probe to the smallest planet: to get close-up images and take all sorts of data which will help us understand this hot, dense world.

MESSENGER is doing a series of gravitational loop-de-loops to get to Mercury, and has passed the planet three times already. In October 2008, during its second flyby, it took this astonishing picture. It shows two prominent fresh craters on the airless planet, but also a series of vast, world-spanning rays: plumes of material ejected from an impact. Their existence had been inferred from earlier observations, but this was the first time they had been directly seen. We'll learn a whole lot more about Mercury when MESSENGER finally settles into orbit in March 2011.

Related post: Watermelon Planet

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Venus

In 2004, I was able to witness an almost literally once-in-a-lifetime event: the Transit of Venus across the Sun. Because of the odd geometry of our sister planet's orbit, it crosses the Sun's face in pairs: one transit following the other after a period of about 8 years, but then no other for over a century. The last pair was in 1874 and 1882. We're in the middle of a pair right now; the last was in 2004, and the next in 2012.

While I watched the 2004 transit myself with my own eyes, NASA's solar-observing TRACE satellite saw it as well. Pictured above, you can see the transit in visible light (top) -- scattering of sunlight by the thick atmosphere of Venus makes it look like a complete ring -- ultraviolet (bottom left) and the far UV (bottom right). Astronomers were able to learn about Venus's air during this event. Also, planets around other stars have been detected when they transit their stars, so observations like this in our own solar system give us insight into the physics of these events.

Credit: NASA/LMSAL

Earth

There's nothing like a home picture, is there? This remarkable shot was taken by the European Space Agency's probe Rosetta, which will rendezvous with a comet in 2014. It needed a little gravitational assist to get there, so it swung by the Earth three times (and Mars once). When it was still over 600,000 km (360,000 miles) from Earth on the third pass in November 2009, it snapped this incredible picture of our home planet. It was still approaching at the time, coming in from an angle that made the Earth appear to be a thin crescent.

At closest approach, Rosetta skimmed the Earth at a distance of just 6000 km (3600 miles) above the surface! Close enough to feel the breeze from it... if it weren't for that whole "vacuum of space" stuff. As it was, the spacecraft accelerated by about 13 km/sec (8 miles/sec), enough to boost it on its way to meet up with a comet. When it arrives, it will deploy a lander that will touch down on the surface of the comet and study it up close and personal, giving us our best view yet of these objects.

Related post: Rosetta Takes some home pictures

The Moon

I know, the Moon's not a planet, but it's big and close and cool, and I love this picture. It shows the rim of the crater Erlanger, located almost at the Moon's north pole at a latitude of 87°. From that location the Sun is perpetually on the horizon, so the crater floor is never illuminated. The rim, however, sticks out above the rest of the surface, and can be lit up by the low Sun.

The crater is about 10 km (6 miles) across, and is a candidate location for ice frozen under the surface. Scientists have recently discovered that the Moon has quite a bit of water ice trapped under the surface dust, and places like Erlanger -- which never see the warming rays of the Sun, even after billions of years -- may have huge reservoirs of water eternally frozen at their bottoms. This would make Erlanger a good place to have a lunar base: water is abundant, and solar cells along the rim would deliver power 24 hours a day -- sorry, I mean 655 hours a lunar day.

Related post: Martian Swirly

Credit: NASA/JPL/University of Arizona

Jupiter

In 1997, NASA launched the Cassini spacecraft on its way to Saturn. To get there (like MESSENGER and Rosetta in the Mercury and Earth pictures in this gallery) it needed a little help. So in 2001 Cassini passed by Jupiter, stealing a little bit of Jupiter's energy and boosting itself to a higher speed. It didn't get all that close to the big planet -- 10 million kilometers (6 million miles), or 25 times the distance of the Moon from the Earth -- but its powerful cameras were able to take this stunning half-Jupiter shot.

This is actually a mosaic of 27 images! It took a 3x3 picture grid of the planet, then repeated it twice to get it through red, green, and blue filters. That way, astronomers back home could stitch them together to make this beautiful and moody true-color picture of the solar system's biggest planet. The detail on the original are incredible; you can see hundreds of small storms raging across the planet, as well as subtle colors and other features. Remember: Jupiter is 86,000 miles across, 11 times the diameter of Earth! Keep that in mind when you see something in this picture that looks "small".

Related post: Cassini: ten years since Jupiter

Credit: NASA/JPL/SSI

Saturn

By now you've probably figured out that I'm partial to crescent and gibbous (that is, more than half full) planet pictures. That's because from Earth, the only two planets we see as crescents are Mercury and Venus; the outer planets are always "full" because of the geometry of the way they're lit from our viewpoint. So there's something particularly compelling about a partially lit planet...

And that's why I love this Saturn shot from Cassini. Taken from high above the plane of the rings, Saturn is a little more than half full. The rings appear darker than usual, and that's because on the day before this picture was taken, Saturn experienced its spring equinox. That means the rings were pointed edge-onto the Sun. Instead of the sunlight falling on the rings, illuminating them, it was hitting the edge. So the rings appear dark, and with Saturn half-lit the way it is, this picture is more brooding than most cheery, well-lit pictures of the ringed planet. Sure, the famous shot of the back-lit planet with the Earth peeking between the rings is more famous, but this one has a depth and a color to it that really appeals to me.

Behold Saturn!

Top 10 Astronomy Pictures of 2007: runners up

Happy Birthday Cassini - and thanks for the great images!

Credit: NASA/JPL/Space Science Institute

Uranus

Uranus is a pretty interesting place. At a distance of three billion km (2 billion miles) form the Sun, it's actually visible to the naked eye under excellent conditions. But even so, we've only really begun learning about it relatively recently. Its rings were discovered in 1977, and directly seen for the first time in 1986, by the Voyager 2 probe.

The image above is from Hubble Space Telescope's NICMOS camera, which can see infrared light. It shows the planet, of course, and its rings, but also 10 of the giant planet's moons, as well as an incredible band of storms raging across the cloud tops. The colors of the clouds indicate depth: blue comes from deeper clouds (methane in the atmosphere absorbs red light-- the same reason deep water looks blue), yellow and gray from high clouds and haze, and the orange and reds from extremely high clouds. Also note the angle of the planet: it orbits the Sun tilted over "on its side", so even from Earth we can trace the rings circling all the way around the planet.

Credit: Erich Karkoschka (University of Arizona) and NASA

Neptune

Voyager 2 passed Neptune in the late 1980s and returned awesome pictures. While the one I decided to post here may not grab you as instantly as those would have, I wanted to use it because I think it's really cool. It was taken by the New Horizons probe, a relatively small but ambitious mission that is sending the probe flying past Pluto in 2015.

Pluto crosses Neptune's orbit, but due to the timings of their motion they never get very close; Pluto is in no danger of crashing into Neptune. So this picture taken by New Horizons is from a long way off: 4 billion kilometers, in fact! Neptune actually gets closer than this to Earth sometimes... which may give you an idea of just how far away this spacecraft is. The shot shows Neptune (overexposed in the middle) as well as its frozen moon Triton. Pluto and Triton have quite a bit in common -- they're about the same size, temperature, and have the same atmospheric composition -- so this was a good practice shot for the mission. It also gives me a lot of confidence that when it does pass by Pluto, we'll get some amazing pictures.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Pluto

I know, I know. Pluto's not a planet, blah blah blah. Well, a lot of people still hold Pluto dear in their hearts so that's a good reason to include it. And if you prefer, then think of this entry as being an example of a big Kuiper Belt object.

Either way, the image here was an awesome achievement: amateur astronomers on Earth were able to take pictures of Pluto that actually show its moon Charon! Given that the moon wasn't discovered until 1978, by a professional astronomer using a big telescope, getting this shot really was an incredible accomplishment. Amazingly, the telescope used for this image was a 35 cm (14"), far smaller than the one used to discover Charon, and in fact this image is far superior! We've come a long, long way in the past 30 years.

Charon image credit: Coelum Astronomia, Daniele Gasparri, and Antonello Medugno

Bonus: exoplanet Fomalhaut b

I can't help it, I have to throw in one more.

When I was a kid, there were 9 planets. We really didn't know if other stars had planets circling them or not. Today, we now know of hundreds of these exoplanets, detected using various means. But it wasn't until 2008 that we finally clutched the Holy Grail: a bona fide, 100% confirmed direct image of one of these planets.

The image above is from the Hubble Space Telescope. It was observing the bright star Fomalhaut, which you actually can't see in the image because its light was blocked purposely so that fainter material around it could be seen (just like when you look for an airplane near the Sun and block the sunlight with your hand). The ring is a vast torus of dust leftover from the formation of the system, and we knew from its shape there might be a planet near it. And sure enough, an image from 2006 was compared to one taken in 2004, and a moving dot was found: the planet Fomalhaut b. It orbits the star at a distance of 16 billion km (10 billion miles), much farther out than Neptune is from the Sun. That's why we could see it at all; had it been much closer it would be lost in the glare of the star, a billion times brighter.

I love this picture (as well as another released at the same time of more planets orbiting a different star): it is solid evidence that we are learning more about our Universe everyday, and that questions we have had for centuries, for millennia, are answerable if we put our considerably clever minds to them.

Ad astra!

Another direct picture of a planet orbiting an alien star confirmed!

Credit: NASA, ESA, P. Kalas, J. Graham, E. Chiang, E. Kite (University of California, Berkeley), M. Clampin (NASA Goddard Space Flight Center), M. Fitzgerald (Lawrence Livermore National Laboratory), and K. Stapelfeldt and J. Krist (NASA Jet Propulsion Laboratory)