Like Tatooine in ‘Star Wars,’ this planet has two suns

Fans of Star Wars might remember watching a moody Luke Skywalker gaze at a double sunset on his home planet of Tatooine. It turns out that planets with two suns probably are more common than once thought. Scientists recently discovered the tenth such planet. And they say it adds to evidence that such planets may be more common than single-sun ones like Earth.

Scientists have known for a long time that most stars come as pairs or multiples. They wondered if these multi-star systems might also host planets. After the Kepler space telescope was launched in 2009, astronomers finally had the tools to search for these among exoplanets. Those are worlds outside Earth’s solar system.

The newfound exoplanet, Kepler-453b, is 1,400 light-years from Earth. It orbits in a two-sun — or binary — system. Planets in such a system are called “circumbinary” for the fact that they circumnavigate both stars.

Astronomers discovered Kepler-453b while watching two stars that were orbiting each other. Sometimes light coming from the stars dimmed a bit.

“That decrease has to be because of something going in front of the stars,” explains Nader Haghighipour. He’s an astronomer at the University of Hawaii at Manoa. He was one of the authors of an August 5 paper about the planet’s discovery in Astrophysical Journal.

He shared details of this planet and star system on August 14 at the International Astronomical Union General Assembly in Honolulu, Hawaii. And something was unusual about the new circumbinary planet. Of the other nine such planets known, eight orbit on the same plane as their stars. That means they pass in front of both stars every time they make a complete orbit. But the new planet’s orbit is tilted a small bit compared to the orbit of its suns. As a result, Kepler-453b only passes in front of its stars about 9 percent of the time.

“We were really lucky,” says Haghighipour. If his team hadn’t been watching the stars at just the right moment, the scientists would have missed the telltale dip in light that signaled the presence of this planet.

That they found this planet at all ­— the second circumbinary planet with such an off-plane orbit — probably means that they’re incredibly common, the astronomers say. Indeed, Haghighipour adds, “We realized there must be many other systems that we’re missing.”

After all, if a planet’s orbit never allows it to pass between Earth and its stars, no telltale dip in starlight will ever point to the planet’s existence. The next step will be for astronomers to figure out how to detect these types of planets. Haghighipour thinks its possible. If the planet is big enough, its gravity will affect its stars’ orbits. Astronomers could search for those tiny, telltale wobbles.

Most known exoplanets orbit a single star. But that’s partly because of observational bias, notes Philippe Thebault. He’s a planetary scientist at the Paris Observatory in France. He was not involved in this discovery. Early exoplanet surveys excluded systems with multiple stars. Even after scientists started looking at two-star systems, they found that most of the planets that turned up were orbiting only one of the two stars.

Some exoplanets have even more suns. A few orbit in three- and even four-star systems.

Thebault says more circumbinary systems need to be studied. That way, scientists can learn more about how they work and how common they are. “It’s still difficult to do statistics” to figure that out, he says. There are simply too few examples known. He says, “It will be nice to have 50 or 100 of these guys, instead of 10.”

So is it possible there’s a young Jedi watching a double-sunset over Kepler-453b today? It does reside in the habitable — or “Goldilocks” — zone. That is the distance from a sun that allows water to be liquid and the planet’s surface not too hot to fry life or too cold to freeze it. Life on Kepler-453b might be unlikely, though, since this exoplanet is a gas giant. That means it has no solid surface. But it could have moons, Haghighipour says. “Such a moon [would also be] in the habitable zone, and may develop conditions to start and sustain life.”

Power Words

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astronomy    The area of science that deals with celestial objects, space and the physical universe as a whole. People who work in this field are called astronomers.

astrophysics   An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.

binary  Something having two integral parts. (astronomy) A binary star system contains two suns in which one revolves around the other, or they both revolve around a common center.

circumbinary   (in astronomy) An adjective that describes a planet that orbits two stars.

circumnavigate   To travel all around something, such as to complete at least one orbit around a star or to travel all of the way around the Earth.

exoplanet  A planet that orbits a star outside the solar system. Also called an extrasolar planet.

Goldilocks zone   A term that astronomers use for a region out from a star where conditions there might allow a planet to support life as we know it. This distance would be not too close to its sun (otherwise the extreme heat would evaporate liquids). It also can’t be too far (or the extreme cold would freeze any water). But if it’s just right — in that so-called Goldilocks zone — water could pool as a liquid and support life.

gravity  The force that attracts anything with mass, or bulk, toward any other thing withmass. The more mass that something has, the greater its gravity.

habitable  A place suitable for humans or other living things to comfortably dwell.

light-year  The distance light travels in one year, about 9.48 trillion kilometers (almost 6  trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.

orbit  The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.

plane    (in geometry) A flat surface that is two-dimensional, meaning it has no surface. It also has no edges, meaning it extends out in all directions, without ends.

planet  A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and it must have cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, it must be big enough to pull neighboring objects into the planet itself or to sling-shot them around the planet and off into outer space. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. Based on that definition, IAU ruled that Pluto did not qualify. The solar system now consists of eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

solar system  The eight major planets and their moons in orbit around the sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.

star  Thebasic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

statistics The practice or science of collecting and analyzing numerical data in large quantities and interpreting their meaning. Much of this work involves reducing errors that might be attributable to random variation. A professional who works in this field is called a statistician.

sun  The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Or a sunlike star.

telescope    Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.