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SYFY WIRE Bad Astronomy

Opposing Jupiter: Hubble's view of the largest planet in the solar system

By Phil Plait

If you go out tonight just after sunset and look east, you will see a bright star rising. Except that’s no star: It’s Jupiter.

Tonight, Friday April 7, 2017, at 21:30 (or so) UTC, the Sun, Earth and Jupiter fall very nearly along a straight line in space. We call that opposition, because the Sun and Jupiter are opposite each other in the sky.

It’s also the time when Jupiter is closest to Earth during the year —think of them as race cars going around a track at different speeds, with Earth in the faster inside track. The two are closest together when Earth passes Jupiter on the inside, and they fall along a line connecting them to the Sun (see the diagram on this post, which is about Uranus, but you’ll get the point). At opposition, Jupiter was 666.5 million kilometers (417.5 million miles) from Earth.

Since that’s as close as it’ll be all year, that means Jupiter looks as big as it will all year. Right now it’s a smidge over 44 arcseconds across (an arcsecond is one-sixtieth of an arcminute, which in turn is one-sixtieth of a degree; the Moon is 0.5 degrees = 30 arcminutes = 1800 arcsecond across for comparison). Through even a small telescope, the planet will look wonderful, with its stripes and four bright moons easily visible.

So, you can imagine what it would look like through Hubble. Oh wait, you don’t have to imagine it! Taking advantage of this near-annual event, astronomers pointed the orbiting observatory at the King of the Planets, and what they got back was jaw-dropping awesomeness.

Jupiter via Hubble

How about that? The Great Red Spot, that swirling centuries-old anticyclone, dominates the lower left quadrant, but other Earth-sized storms are easily visible dotting the clouds. And those clouds! The dark belts alternate with lighter zones, regions where upwelling warm gases inside the planet create circulation patterns that stretch all the way around the cloud tops. As the rising air cools, ammonia ice forms in it. This is light in color, forming the zones. The air sinks as it cools, moving to the sides, at the same time as sunlight interacts with the molecules there, darkening them. This forms the belts.

They circulate around the planet in opposite directions, and where they meet, there’s turbulence, which you can see as swirls and festoons at the borders. This image, taken April 3 just days before opposition, shows all this in startling clarity.

I’ll note that the colors in the image aren’t quite what we’d see with our eyes were we floating over the planet in space. A red, green, and blue filter were used to approximate natural colors, but the filters only let through a very narrow range of those colors, while our eyes are sensitive to a wider swath of each part of the spectrum. I suspect this accentuates the contrast in the cloud coloring a bit, making it look sharper and allowing more detail to be seen.

If you want to learn more about Jupiter, may I not-so-humbly suggest taking a few minutes to watch this episode of Crash Course Astronomy on it?

After you’ve seen that, go outside after dark and take a look. Jupiter is up literally all night long (it’s opposite the Sun in the sky, remember, so it rises when the Sun sets and sets when the Sun rises), so you have plenty of time to take a peek. And it’ll look great for many weeks, rising a bit earlier every night, making it the favorite target for astronomers all over the world. If you live near an observatory, or there’s a local astronomy society/club in your area, seek them out. Jupiter is wondrous to see, and this’ll be your best chance until May 8, 2018: the next time we lap the gas giant in our orbital race.