Meet our Solar System’s twin

Our Solar System is an oddball. Of all the 1,200-plus planetary systems found so far, none have had an arrangement of planets like ours. Now, 200 light years away, we may have found our Solar System’s twin. Astronomers have discovered a star almost identical to our Sun, orbited by a planet just like Jupiter – and they’ve also detected hints the system could harbour small rocky planets in habitable inner orbits.

Studying such systems could shed new light on the formation of our own Solar System – and help us determine whether life exists elsewhere in the Universe. The discovery, announced in July, will be published in the journal Astronomy and Astrophysics. "The quest for an Earth 2.0 and for a complete Solar System 2.0, is one of the most exciting endeavours in astronomy," says Jorge Melendez of the University of Sao Paulo, who led the work.

“The quest for an Earth 2.0, and for a complete Solar System 2.0, is one of the most exciting endeavours in astronomy.” 

But finding distant planets has so far proved difficult, says Chris Tinney, an exoplanet astronomer at the University of New South Wales. “Stars are incredibly bright and planets are ludicrously dim,” he explains. This means seeing exoplanets directly is almost never possible.

So astronomers have developed several ways to detect exoplanets indirectly. The classic method, first used in 1989, is to detect a star’s wobble caused by the tug of a planet in its orbit. The wobble causes a star to appear to move towards and away from us in a regular pattern.

To give themselves the best chance of finding a planetary system similar to our own, Meléndez and his team focused their wobble search on 60 stars known to be similar to the Sun in terms of their luminosity, age and composition. Using a telescope based in La Silla in Chile, the team tried to identify whether any of these systems shared another key feature that makes our Solar System special: Jupiter.

Jupiter has long been considered important to life’s appearance and evolution on Earth. At the least, it may have acted like a big brother to the little rocky planets closer to the Sun, protecting them against bombarding asteroid bullies from the outer Solar System. But Jupiter may have played a more important role. “According to the latest theories, Jupiter is fundamental to the stability of our Solar System,” says Melendez.

Work published by a California-based team in the Proceedings of National Sciences in April suggests Jupiter played a key role in Earth’s formation. Their simulations suggest the gas giant helped corral rocky material in inner orbits around the Sun, where it eventually coalesced to form Mercury, Venus, Earth and Mars. 

While many Jupiter-sized exoplanets are known, most are 'hot Jupiters' orbiting close to their star. Systems with gas giants in a more distant orbit, like our own Jupiter, have been harder to find.

The planet discovered by Melendez and his team, orbiting the Sun-like star known as HIP 11915, is almost exactly the same mass as Jupiter and orbits at the same distance from its parent star.

Whether or not a twin-Jupiter is important for a twin-Earth is still debatable, says Tinney. The formation of each planetary system is probably so complex and chaotic, it’s difficult to draw general conclusions based on the role Jupiter may have played in our own system, he says. “It's like trying to predict the weather. Every cyclone is unique and every planetary system will turn out to be unique.”

Though the team has not yet detected any smaller rocky planets, they have found one tantalising clue they might be there. Our Sun, and HIP 11915, share an unusual chemical composition. Compared to other stars of the same age and size, the Sun is short of iron, nickel and magnesium. But these elements are abundant on Earth. According to one theory, as our Solar System formed these elements accumulated in the rocky material that eventually became Earth and its neighbours.

HIP 11915 also lacks these tell-tale elements – suggesting it, too, might host rocky planets.

Current telescope technology struggles to detect the star-wobble caused by Earth-sized planets, but an upgrade to the La Silla system is taking place. Plus a new exoplanet-spotting instrument for the Very Large Telescope, also in Chile, is planned for next year. They will boost the ability of astronomers to detect planets a little bigger than Earth.

Meanwhile, NASA did announced in late July the discovery of a distant planet that might be similar to Earth, orbiting a star in a different part of the sky to HIP 11915. The planet was picked out by NASA’s orbiting exoplanet hunting telescope, Kepler. The planet, dubbed Kepler 452b, is the first near Earth-sized planet discovered orbiting within the habitable zone of a star similar to the Sun.

But whether Kepler 452b is a rocky planet, or made of gas, is yet to be determined. Tinney, for one, was a little underwhelmed by this announcement. “Kepler has been announcing ‘Earth 2.0’ about once every nine months for the last few years. Each one has been better than the previous one in much the same way each time Usain Bolt breaks the world record, it's a new world record – by a little bit.”

We can expect another new record breaker soon, he says.

Also in Cosmos: A leap forward in the hunt for Earth-like planets