Analogues of our planetary system

The night sky shines with the light of billions of distant stars. Scientists believe that most of these stars have at least one exoplanet. To find these distant planetary systems, astronomers use different methods. The transit method helps detect planets that orbit close to their stars, while direct imaging is better for seeing planets that are farther away. The radial velocity method is very sensitive to large planets, and gravitational microlensing can detect small planets. But how can we find a second Solar system among all these other worlds?

Even back in the 1500s, Copernicus realized that Earth is not a special planet. That’s why he rejected the geocentric model, where the Sun goes around the Earth, and instead proposed the heliocentric model. So why, even with modern technology, haven’t we found a planetary system exactly like our Solar System?

A simple answer might be that the universe is just so huge. It’s big enough that every star and planet can be unique—just like no two snowflakes are exactly the same. But that answer isn’t very satisfying 😉. That’s why it’s worth looking back to 2006, when a special system was found: OGLE-2006-BLG-109Lbc.

Bennett, D. P., et al. 2010, ApJ, 713, 837, Masses and Orbital Constraints for the OGLE-2006-BLG-109Lb,c Jupiter/Saturn Analog Planetary System

This system was discovered via gravitational microlensing. This happens when a hidden planetary system passes between us and a distant star - source. The gravity of the planets bends space-time and causes the source's light to bend toward us. By studying how the star’s brightness changes (called a light curve), scientists estimated the system’s properties — including planet masses of 0.71 and 0.27 times the mass of Jupiter.

What’s surprising is that this system, located about 1.5 kiloparsecs from Earth, looks like a scaled version of our Solar System. It has a star with half the mass of the Sun, and two planets — one like a smaller Jupiter, and the other like a smaller Saturn — orbiting at about half the distance of their Solar System twins.

Usually, microlensing doesn’t allow us to see light directly from the planet system, because these systems are so far away. But 1.5 kpc isn’t too far — especially for a powerful telescope like Keck ;). Thanks to that, the system’s properties were measured more precisely than in most microlensing events.

So we can be fairly confident that a red dwarf star is being orbited by two planets with half the mass of Jupiter and Saturn. But why is this system so interesting, among the nearly 6,000 exoplanets discovered so far?

First, it looks like a mini version of our Solar System, which makes it a rare and exciting find.

Second, some theories suggest that rocky planets are more common in systems that also have gas giants. Big planets can act like protectors, blocking dangerous objects like asteroids from hitting smaller inner planets. Studies of OGLE-2006-BLG-109Lbc suggest that if rocky planets exist there, there may be more than one. Also, gas giants may help deliver water to the inner planets and shape the “habitable zone” — the area where liquid water could exist. Simulations even support the idea that rocky, possibly water-rich planets could exist there.

Third, this system raises an interesting question: how can such large gas giants form around a star that is so small?