Planet formation

PLANET SYSTEMS

2The next richest planetary system we know of is that around the dwarf star KIC 11442793, 2,500 light years away, which comprises seven planets in tight orbits. 3 The Gliese 581 system, located about 22 light years away, contains at least four planets, and we think that one or two of these might be potentially habitable. 4 The planetary system around Gliese 667 is interesting in that it is a triple-star system around which at least six planets are in orbit, and possibly one more.

5A debris disc similar to what we think was present in the prime example of a planetary system still in formation. early Solar System suggests the Eta Corvi system is a

DID YOU KNOW?

The first exoplanet was discovered in 1992 – a bizarre world in orbit around a pulsar

Planets in the making

We speak to astrophysicist Eric Ford from Pennsylvania State University about current hypotheses in the field

Can you explain the current understanding of how planets form?

When gas collapses to form a star, a portion of the gas forms into a disc. As the disc cools, some of the iron, silicon, carbon, oxygen and water condenses to form grains. When grains collide, they can stick and grow into pebbles. We’re still unsure of the details, but somehow the pebbles become incorporated into larger bodies that grow to become asteroids, rocky planets and the cores of giant planets.

How has the discovery of exoplanets changed our theories of planetary formation?

The discovery of exoplanets very different to our own has opened our mind to the diversity of planetary systems that nature can produce. The observations have inspired a wide range of theories about how planets of various masses and sizes could be arranged into the configurations that we observe today.

Could our understanding of how the planets formed in the Solar System be wrong?

NASA’s Kepler mission has demonstrated that most planetary systems are unlike our Solar System. Instead of designing planet formation to reproduce our Solar System, now we must develop theories that make a wide variety of planetary systems and only occasionally produce something similar to our system.

What future observations are needed to better understand planetary formation?

We’ll need lots more observations. Fortunately, there are several new facilities in the works that will lead to major advances in our ability to directly image large planets far away from their star and to resolve structures in the protoplanetary discs.

Hopefully, the major space agencies can co-operate to develop at least two more generations of major space-based observatories, so we’ll be able to discover and characterise Earth-like planets.

Eccentric orbits

The less circular a planet’s orbit is, the more eccentric it is said to be. Earth’s orbit, with the Sun at its centre, has an eccentricity of 0.0167, with 0 being a circular orbit and 1 being parabolic. Some exoplanets, however, have highly eccentric orbits, with their host star focused towards one end. One of the most eccentric planets we know of is HD 80606b, located in the constellation of Ursa Major, at 0.93. Its orbit takes it from a distance of just 5 million kilometres (3.1 million miles) from its host star right out to 130 million kilometres (80 million miles), with its speed increasing as it gets closer and vice versa.