These planets are unusual because they orbit, or move around their stars, backward. In the solar system, all eight major planets (sorry, Pluto!) move around the sun in the same direction: counter-clockwise when looking down on the sun’s north pole. The sun, too, is spinning in that direction.

Scientists believe that all the planets in the solar system were formed from the same giant disk of debris — mainly gas and dust — that was slowly moving around the sun billions of years ago. Since the debris was moving, the planets, including Earth, that formed also moved in the same direction as the debris.

This image is from a video that illustrates a planet in retrograde orbit: The star is spinning to the right and the planet is rotating to the left. See the European Southern Observatory’s video.

ESO, L. Calçada

In addition, the paths of all the planets should be in the same plane. Imagine a giant, flat piece of paper that cuts through the middle of the sun and extends to the end of the solar system. If all the planets orbit in the same plane, then all their orbits will be on that piece of paper.

That’s the way it works in the solar system, so astronomers have wondered whether planetary systems around other stars work in the same way.

Last summer, astronomers found six planets moving around their host stars in the opposite direction. This finding suggests that scientists may have to think again about how planets form. All six of these planets are “hot Jupiters.” Hot Jupiters are giant — as big as or bigger than Jupiter — and orbit so close to their host stars that they’re blazing hot.

Illustrated here are a few planets that orbit their stars in the wrong direction. The lower right image shows a planet orbiting in the same direction its parent star rotates.

ESO, L. Calçada

These six aren’t the only problematic planets. Some other recently discovered planets orbit in the forward direction around their host stars, but their orbital planes tilt at various angles.

At a recent meeting of astronomers in Glasgow, Scotland, Andrew Collier Cameron proposed an explanation for these wrong-way and tilted planets.

Cameron, an astronomer at the University of St. Andrews in Scotland, suggested that a much larger object —another star, or a giant planet, perhaps — may have come along. Gravity is a force that comes with mass, so planets or stars with more mass have more gravity, and thus a stronger pull on other objects. Large objects have strong gravitational forces, and these strong forces may have affected the way the planets move around their stars. Astronomers believe these forces can be so strong that they cause the planet’s orbit to flip like a jump rope over the star.

This effect, called the Kozai mechanism, may explain how a hot Jupiter ends up orbiting backward around its star. It may also explain how the other planets ended up with tilted orbits.

Cameron says the wrong-way planets match up with the change his team would have expected from the Kozai mechanism. “That looks very much like what we’re now observing,” Cameron says. “It looks almost too good to be true.”

Other scientists say it’s too early to say for certain whether the Kozai mechanism is responsible for the planets’ behavior. “Their data isn’t that definitive to eliminate any other possibilities,” Adam Burrows told Science News. Burrows is a scientist at Princeton University.

Astronomers have identified more than 400 exoplanets, and most of them are gas giants, like the hot Jupiters. (Exoplanet is short for “extra-solar planet,” which is a planet outside the solar system.) Astronomers would like to find a small, rocky planet not too far from or too close to its star — one that looks a lot like Earth. These types of planets are most likely to host life as we know it, so if we find an Earthlike planet, we may find life somewhere else in the universe.

Then again, we may not. If Cameron is right, then hot Jupiters on strange orbits may fling rocky debris — debris that would have made a small planet — out of the system. So in a way, more hot Jupiters may mean fewer Earthlike planets.

More studies are needed to know for sure why some planets run backward around their host stars.

Going Deeper:

Grossman, Lisa. 2010. “Backward planets may have flipped into place,” Science News, April 13. http://www.sciencenews.org/view/generic/id/58275/title/Backward_planets_may_have_flipped_into_place

Cowen, Ron. 2009. “Extrasolar planets at full tilt,” Science News, Sept. 12. http://www.sciencenews.org/index/generic/activity/view/id/46658/title/Extrasolar_planets_at_full_tilt
Ornes, Stephen. 2010. “Cool as a Jupiter,” Science News for Kids, April 7. http://sciencenewsforkids.org/articles/20100407/Note1.asp
Read all about the quest to find another Earth at NASA’s Planet Quest web site: http://planetquest.jpl.nasa.gov/

Each white dot represents an individual piece of tracked orbital debris. This image shows the Low Earth Orbit, which is the region from the Earth’s surface to 1,240 miles and contains the most space junk.

On a clear night, you can look in the sky and see the moon and stars. You might even see the blinking light of a working satellite as it flies past, on its way around the Earth.

And, even though you cannot see it, you are also looking at the largest junkyard in the solar system.

Higher than the highest clouds but much closer than the moon, the bulk of the junkyard stretches from the Earth’s surface to 20,000 miles overhead. There are tens of millions of pieces of rubbish there. Some of the pieces are rocks and dust from passing comets, but most of them are manmade and called “orbital debris” (pronounced duh-BREE).

There are some unusual things up there, like a camera that floated away from astronaut Sunita “Suni” Williams in December 2006. Other astronauts have lost tools like wrenches and screwdrivers. In 1965 astronaut Ed White even lost a spare glove. Most of the junk, however, comes from large satellites and rockets that fell apart after they stopped working.

Together, all the space junk would weigh about 11 million pounds on Earth, or more than 3,000 cars. The largest piece is a part of a rocket about the size of a minivan. The smallest piece would fit on your pinkie fingernail with room to spare.

“It’s like a classic environmental problem, like water pollution or air pollution,” says Nicholas Johnson. Johnson is the Chief Scientist for Orbital Debris at the NASA Johnson Space Center in Houston. His job is to keep track of the orbiting garbage.

The junkyard is a serious problem for the future of spaceflight.

“You’ve got multimillion satellites in orbit all the time, and manned space flights,” says Joe Gambrell, who helps keep track of both working and broken-down satellites. His main tool is the U.S. Space Surveillance Network, which is part of the U.S. military. Gambrell works at the Peterson Air Force Base in Colorado Springs, Colo. “If you don’t track debris, you risk some kind of collision,” he adds.

In 2007, the space junkyard grew by more than 100,000 pieces. That’s more than any other year since people started launching satellites into space. The problem of space junk is not going away, and scientists are watching closely.

Danger in Orbit

A “satellite” is any object that orbits another object, held close by gravity. The moon is a satellite of the Earth, and the Earth is a satellite of the sun. These are natural satellites. Manmade satellites, which are built on Earth and launched into space, are used for communications, scientific studies and military applications.

In the last 50 years, human beings have launched thousands of artificial satellites into space. When a satellite stops working, it usually falls back toward the Earth and burns up in the atmosphere. Satellites at high altitudes, however, sometimes remain in Earth’s orbit.

Later, they may fall apart or explode into thousands of smaller pieces. The higher the satellite, the longer it stays in orbit, and the more likely it is to break apart. The pieces may stay in orbit for years, decades or even centuries.

At the NASA Ames Research Center, scientists simulate what would happen were a tiny piece of space junk to hit a spacecraft.

NASA

Space junk races around the Earth at breakneck speeds. Most pieces fly through space at more than 20 times the speed that sound travels on Earth. Going that fast, even the smallest pieces mean big trouble for spacecraft. For example, a tiny marble in orbit around the Earth can have as much energy as a bowling ball going 500 miles per hour, or a car going 30 miles per hour.

A tiny fleck of paint struck and cracked one window of the space shuttle Challenger while it was in space in 1983. The shuttle returned to Earth safely from that mission.

NASA Johnson Space Center

In 1983, a small gouge appeared in one window of the space shuttle Challenger while it was in space. When the shuttle returned to Earth and scientists analyzed the window, they found that the crack was caused by a tiny, orbiting fleck of paint. If the shuttle had been struck by a larger piece of junk, the astronauts may have been in danger.

Many other satellites and space shuttles have also shown damage from tiny pieces of trash. When the space shuttle Endeavour returned to Earth last August, its radiator panels had small holes from space garbage. Last year, two satellites had to be redirected to avoid collisions with big pieces of junk.

The U.S. Space Surveillance Network and NASA work together to keep track of the largest pieces. When the shuttle is in orbit, for example, their attention is on nearby junk that may get in the way. If there is even a small chance of a collision, then the shuttle changes direction.

Watching the garbage

About 17,000 pieces of garbage are larger than 4 inches, which is slightly smaller than a softball. The Space Surveillance Network keeps a list of all these pieces, using dozens of telescopes and antennas on Earth and in space to watch them. Because all the pieces are in motion all the time, keeping track is difficult (but not impossible).

Monitoring the space junk is a problem of energy and motion. To keep track of the junk, the scientists have to know two things: where the garbage is now, and where it is going to be in the future.

This antenna, located in the Mojave Desert, California, can use radio to detect tiny pieces of junk, if they’re not too high.

NASA Johnson Space Center

To find out where a piece is now, scientists use telescopes and high-powered antennas to watch it fly through space. Every day, scientists must make sure each piece is where they expect to find it. A “lost piece” could mean serious danger for a working satellite or space shuttle.

To find out where the piece is going, scientists measure its speed and direction as it crosses the sky. Some of the pieces move in an almost perfect circle around the Earth. Other pieces move in elliptical orbits, which means they are sometimes closer to Earth and sometimes farther as they fly. The measurement of direction is complicated because junk can move north or south, east or west, and up or down.

By knowing the location, speed and direction of each big piece of junk, the scientists can predict where all the pieces will go. (Similarly, in baseball, an outfielder has to predict where a pop fly will land, if she wants to catch it. In soccer, a goalie has to predict where a ball will go, so he can block it.)

“We’re always looking about four days into the future, tracking objects which might come close,” says Johnson.

There are tens of millions of pieces smaller than softballs. Because there are so many small pieces, “We can’t [keep track] of them all,” Johnson says. Instead of watching each piece individually, scientists use telescopes and antennas to watch one patch of sky and count the number of pieces that pass overhead. With that small measurement, they can use a computer program to get a good idea of what the whole sky looks like.

Keeping track of all the junk is necessary to ensure that astronauts and working satellites can be safe. Like other environmental problems, space junk will get worse without careful monitoring and attention. In the future, pieces of junk will probably hit each other, making even more trash.

The more garbage that remains in orbit, the greater the risk to space flights. What can be done about the problem of space junk? The large pieces of junk cannot be brought back to Earth, Johnson says, because the technology is too expensive. Instead, he says, we should try to stop adding new garbage. For example, engineers are changing the way they build spacecraft.

“How you build your satellite can minimize debris,” Gambrell says.

A solution will require international cooperation. Last year, the United Nations General Assembly approved guidelines for how to reduce the risk of space junk. Johnson says that spacefaring countries came together because “the environment was getting worse every year.”

If countries work together to control the amount of trash we send into space, Johnson says, we can keep the problem under control. In the future, perhaps we can even clean up the mess.

“In time, either technology or economics will allow us to go out and [improve] the environment,” he says.

Going Deeper:

Try it. Next time you’re outside on a clear night, look up. You might see the winking lights of an airplane, the glow of an orbiting satellite, or even the bright trail of a meteor. Of course, you’ll see lots of stars.

What about all the space between the stars? Is something hidden out there in the darkness? Or is it merely empty?

Is there anything in the dark areas between distant galaxies?

NASA, ESA, the GOODS Team, and M. Giavalisco (STScI)

There’s nothing for the human eye to see, but astronomers are finding ways to detect what lies between the stars. And they’re discovering that most of the universe is made out of mysterious, invisible stuff. They call it dark matter and dark energy.

Although they can’t see it directly, scientists are pretty sure this weird stuff exists. Figuring out exactly what it is, however, remains a work in progress.

“We’re just now beginning to peel away the darkness,” says Robert Kirshner, an astronomer at Harvard University. “We’re beginning to see what things are really like, and it’s a funny, very unsettling picture because it’s so new and unfamiliar.”

Ordinary matter

When you look around, everything you see is a type of matter. This is the ordinary stuff of the universe, from a grain of salt to a drop of water to a candy bar. You are matter. So is Earth, the moon, the sun, and our own Milky Way galaxy.

Simple enough, right? Until about 1970, our picture of the universe seemed this straightforward. But then Jeremiah Ostriker of Princeton University and other astronomers started to notice something curious.

Gravity provided the hint. The force of gravity keeps us stuck to the ground, the moon in orbit around Earth, and Earth in orbit around the sun. Without gravity, these bodies would fly off on their own.

In general, the force of gravity between any two objects depends on the distance between them and on the amount of matter, or mass, in each object. The sun, for example, contains a lot more matter than Earth, so it has a much larger mass and exerts a much greater gravitational force than Earth.

Astronomers can estimate how much ordinary, visible matter a star or a galaxy contains. They can then figure out how the gravity of, for example, one galaxy would affect another, nearby galaxy.

Billions of years from now, the Milky Way galaxy and the neighboring Andromeda galaxy might collide, pulled together by the force of gravity. In this illustration, an artist shows what gravity would do to the crashing galaxies, twisting them out of shape

NASA and F. Summers (Space Telescope Science Institute), C. Minos (Case Western Reserve University, L. Hernquist (Harvard University).

When astronomers compared their calculations to what really happens in our own galaxy, they were surprised to find that the Milky Way acts as if it has much more mass than it should. It’s like going to the carnival where someone tries to guess your weight from your appearance and finds that you weigh 1,000 pounds instead of 100 pounds when you step on the scale.

Measurements of other galaxies produced the same puzzling result.

Out of darkness

The only logical conclusion, Ostriker says, was that there’s lots of stuff out there that’s invisible yet still has mass. Scientists named it “dark matter.” Ordinary matter can give off or reflect light; dark matter does not.

Even then, the concept was too baffling for many people to believe at first, Ostriker says. “But every measurement you make gives the same answer,” he says. “Now, we have to believe it.”

Indeed, calculations show that there may be 10 times as much dark matter as ordinary matter in the universe. The part we see is only a small fraction of all the stuff in the universe.

So what is dark matter? “We have no more clue now than we did 30 years ago,” Ostriker says.

Scientists have been trying out all sorts of ideas. One idea is that dark matter is made of teeny-tiny particles that give off no light, so they can’t be detected by telescopes. But it’s hard to decide what sort of particle fits the bill.

“Right now it’s a lot of guesses, and it’s highly uncertain,” Ostriker says.

Astronomers need more help to figure out what dark matter is. You might end up working on this puzzle yourself if you study astronomy or physics. And if that puzzle isn’t challenging enough for you, there’s more.

Another force

Once astronomers accepted the idea of dark matter, another mystery turned up.

According to the Big Bang theory, the universe started with a huge explosion that pushed all the stars and galaxies away from each other. Based on their measurements of matter and dark matter, scientists concluded that gravity should eventually reverse this motion. It would make the universe collapse back in on itself billions of years from now.

Observatories such as the Hubble Space Telescope (HST) and the Chandra X-ray Observatory can look back into time, detecting light and other radiation that started out from stars and galaxies billions of years ago. Future telescopes, such as the James Webb

NASA and Ann Feild (STScI)

It came as a huge surprise, then, when powerful telescope observations revealed that just the opposite seems to be happening. By measuring and analyzing light from distant exploding stars called supernovas, astronomers discovered that it looks as if the universe is expanding outward faster and faster.

This shocking discovery suggests that the universe has some sort of additional force that pushes stars and galaxies apart, countering gravity. And the effect of this mysterious force must be larger than that of all the matter and dark matter in the universe. For lack of a better name, scientists call this effect “dark energy.”

So, the bulk of the universe is not stars and galaxies and planets and people. Most of the universe is other stuff. And a lot of this other stuff is something very strange called dark energy.

“Now that’s a really weird picture,” Kirshner says. “In a way, you could say that in the last 5 years, we’ve stumbled into two-thirds of the universe.”

Researchers are now hard at work, using telescopes on the ground and in space to look for clues that would tell them more about dark matter and dark energy.

Another view

What’s the point of studying stuff that we can’t even see?

Just thinking about dark matter and dark energy separates us from other animals, Ostriker says. “When you pick up a rock and see little creatures scurrying around, you can say, ‘What do they know about life except what’s under that rock?’” We, on the other hand, can try to understand the universe outside of us, he says.

That can give us a new perspective, Kirshner says.

We can take pleasure in the fact that we’re made from a very small minority of the kinds of stuff that exist in the universe, he says. Studying dark matter and dark energy gives us a sense of how valuable and unusual this “ordinary” sort of matter is.

So, there’s a lot more to darkness than meets the eye, and it’s worth taking a closer look.

Going Deeper:

Word Find: Dark Universe

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