Earlier this month, scientists made new discoveries of two new planets orbiting dual stars and suggested that such planets might be common. What's stranger still is that two suns might not be the limit.

In fact, scientists already know that there are solar systems containing many more stars out there. HD98800, a system about 150 light years from Earth, has two pairs of double stars with one set surrounded by a ring of space dust. And then there's Castor, located in the constellation Gemini. Although it appears in the sky as a single point of light, Castor is actually a system made of six stars: three sets of binary stars, all whirling around a common center of mass.

Astronomers haven't found planets in systems such as these—yet. "There's no reason you couldn't have planets around multiple stars," SETI astrophysicis Laurance Doyle says. But the pulls of many different orbits would make it difficult. "Everything would have to balance just right," he says.

A planet in a system like Castor's would be constantly tugged about by the gravity of its many stars. Imagine a rope pull in which you're on one side and six burly guys are on the other, says Doyle. They're yanking on the rope, but they're out of sequence, so you can hold your own. But if all six yank at the same time, you've had it, "and the planet goes zooming out of the system," he says.

Though this makes the existence of a planet in a system with many stars unlikely, it doesn't make it impossible. And if there were such a planet, its sunsets would be incredible.

NASA,ESA and G. Bacon (STScI)

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Red Dwarf Planets

Seventy-five percent of the stars in the universe are red dwarfs—relatively small, dim stars. Typically, scientists have discounted any planets that might orbit red dwarfs from the search for life. But this might be a mistake, Doyle says.

Red dwarfs are less than half the mass of our sun, and emit less than 5 percent as much light. So to be in the habitable zone (the distance from the sun at which liquid water can exist), a red dwarf planet would have to be extremely close to its star. For example, he says, if you had a red dwarf star with one one-hundreth the brightness of our sun, its planet would have to be 10 times closer to it than we are to our sun—but such a planet would experience a whopping 1000 times the tidal force. This means that the gravitational pull of the star on the planet, which contributes to the ebb and flow of the ocean here on Earth, would be so strong that the planet would be locked in place, unable to rotate as it orbited. One side would always face the sun; the other would be permanently dark.

This scorching-on-one-side, freezing-on-the-other planet wouldn't be very welcoming to life. "The front burns off and the back burns out, and you can't have anything habitable," Doyle says. But he thinks that there's still a way red dwarf planets could harbor life.

Doyle has done modeling experiments in which he pumps extra carbon dioxide into a red dwarf planet's environment. With enough of a CO2 boost, processes such as the greenhouse effect can take place, creating an atmosphere that holds in and redistributes heat. This could create a planet that's livable in spite of its locked position. If this model is correct, it means that some of the copious red dwarfs in the universe could have habitable planets. To find out more, Doyle says, one new mission of the planet-hunting telescope Kepler will be to search the sky for red dwarf planets.

Weather patterns on a red dwarf planet like the ones in Doyle's model would be incredibly bizarre. Weather patterns emanate from the sun, and here on Earth, they have a sort of hula-hoop formation around our planet because it rotates, he says. But on a tidally locked red dwarf planet, the dim red sun would be a permanent fixture, never moving from its spot in the sky. Weather patterns would propagate downward from it in a spiral pattern, like "a ribbon wrapped around an orange from the top to the bottom," Doyle says. "They would have all sorts of weather that we can't even relate to."

Paul Wiegert, University of Western Ontario, Canada

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Trojan Planets

Imagine gazing into the sky and seeing not only the sun or moon, but also a twin Earth trailing behind us in our orbit around the Sun.

Many celestial objects have these so-called Trojans—bodies that share their orbits. Earth has one, a Trojan asteroid called 2010 TK7. This asteroid falls into a sweet spot where the Earth, moon, and sun are all pulling equally. We have two such sweet spots on the path of our orbit, says William Welsh, an astronomer at San Diego State University: one 60 degrees in front of us relative to the sun, and one 60 degrees behind us.

The closest thing to a Trojan planet in our solar system is Saturn's moon Janus. Janus is named for a Roman god often depicted with two faces—appropriate because it was often mistaken for Epimethus, another moon of Saturn with an orbit almost identical to Janus's. The orbits are so close that for a long time, Doyle says, astronomers thought Janus was one moon with unpredictable behavior. "Nobody could agree on an orbital period," he says. No wonder: It turns out that Janus and Epimetheus's orbits are so close that when the two planets approach each other, they switch places.

Doyle thinks Janus and Epimetheus were probably once one large moon that was hit by a large object and broken in two. Such an event could just as easily happen to a planet, splitting it to create twins that spin through space together.

Lynette Cook / extrasolar.spaceart.org

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Backward Planets

It's thought that planets begin to form in the disc of matter that swirls around a baby star. Because the space dust and gases are all moving the same way, it stands to reason that the planets they become would all rotate the same direction. But planets that break this rule aren't uncommon.

Our own Uranus rotates on its side, its head pointing toward the sun. Because of this, its seasons seem very strange to us Earth-dwellers. On most places of Uranus a day lasts about 17 hours, but because of the planet's extreme tilt, if you were standing on the north pole, a day would last 42 years, followed by a night of 42 years.

Life on Venus would seem even stranger: Venus rotates clockwise (as viewed from above the sun's north pole), while almost all the rest of the planets rotate counter-clockwise. Venus's days are incredibly long: To an inhabitant on the surface of Venus, the sun would rise in the west and set in the east every 116.75 Earth days.

Why did this happen? Doyle says that both Uranus and Venus probably experienced major impacts that knocked their rotations permanently off course. Welsh thinks that even stranger events could cause an alien planet to actually orbit the opposite direction of its fellows.

"It would be very unlikely," Welsh says, "but imagine that one planet gets pulled from the outer solar system into the inner solar system; maybe it gets pulled in over the top in a weird way." Suddenly, a planet is zooming dangerously around the sun like a car going the wrong direction on the highway.

Such a planet, if its existence were possible, would probably live a short life before it crashed into one of its neighbors. But alien worlds might not spin like Earth does. And if they don't, their days and nights look nothing like ours.

NASA/Tim Pyle

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Multiple Habitable Worlds

Temperatures on the surface of Venus reach higher than 860 degrees Fahrenheit. It's just a little too close to the sun to hold liquid water, and so, astronomers consider it to be just out of the habitable zone. "Venus's ocean, if it had one, evaporated long ago," Doyle says.

But if the formation of our solar system had been tweaked just a little, Venus might have liquid oceans just like ours. If Venus had been switched with Mars—in other words, if Venus occupied Mars's orbit— we could have had two habitable planets in our solar system. That's because while Mars has too little atmospheric pressure to hold water, Venus has plenty, Doyle says.

This made Doyle wonder: What is the maximum number of habitable planets a solar system can hold? "Getting multiple planets in the habitable zone is tricky," he says. "Would they destabilize each other?"

Using our own solar system as a guide, Doyle found that up to four planets could fit in our habitable zone, but only in a very strange formation. Four Earth-size planets in our habitable zone would pull on each other too much for all four to be stable. But a huge gas giant, bigger than Jupiter, orbiting the same distance from the sun as Earth orbits could hold four Earth-size moons, all within our habitable zone, he says.

Of course, this is no more than a thought experiment. But one thing holds true: There's no reason habitable worlds have to be anything like Earth. "I think a little imagination is allowed," Doyle says. To scientists like him, dreaming up alien worlds is the biggest kind of thrill. "Another place like home — there's nothing like that," Doyle says.

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