Our Solo System: Exoplanets Reveal How Unique We Are

Rocky planets inside, gas giants outside — we thought our solar system was typical. But the discovery of worlds around other stars has altered that view.

As 1994 drew to a close, a relatively small telescope in France was recording data that would revolutionize our understanding of planetary systems. A year later Michel Mayor and Didier Queloz announced the discovery of 51 Pegasi b, the first planet beyond the solar system to be found orbiting a star like our sun. With a mass about half that of Jupiter’s but an orbital period of just four days, it was unlike anything seen before. It would lead to a transformation of planetary science.

The technique used to discover 51 Peg b relied on detecting the ‘wobble’ of its parent star, caused by gravitational tugging by the planet. Called the radial velocity method, it has improved by an order of magnitude in the years since. In addition, new techniques for planet discovery have been successfully developed, including microlensing (the bending of light passing by the planetary system), direct imaging, and stellar monitoring to detect planetary transits that briefly dim the star. These methods — especially monitoring for transits — have led to the discovery of thousands of planets and decisively answered a key question. Our solar system, with its familiar architecture of small rocky planets surrounded by gas and ice giants, is not typical.

A more common arrangement is for a star to host close-in super-Earths or mini-Neptunes, worlds with properties that are still poorly understood. Their masses fall between those of the solar system’s terrestrial and giant planets. Often, they have orbital periods shorter than Mercury’s and occupy a barren zone in the solar system. Some of the mechanisms that can form such odd-looking planetary systems were anticipated by theorists, but their ubiquity was not.

The study of extrasolar planetary systems has prompted new ideas for how the solar system formed. Planet formation is now understood as a highly dynamic process that can involve both violent rearrangement of planets and long-term evolution of their orbits. Earlier models, in which planets formed more or less in their final orbits, now appear to be not so much incorrect as incomplete. Computer simulations of how planets interact with each other and with the gaseous disk from which they form have played a central role in this change of thinking.

Despite this progress, it is still hard to answer the question of whether the solar system is special. Observationally, we are limited by the fact that it’s still hard to find true Earth analogues — planets that orbit stars like the sun and have masses and orbits similar to Earth’s. Discovering these planets requires next-generation surveys, such as the Terra Hunting Experiment.

We also need to define what we mean by ‘special.’ Looked at closely enough, planets and planetary systems are all expected to be unique. Some of their differences may be unimportant quirks, but many others influence planetary habitability. In our solar system, the Earth’s water content, magnetic field, large moon, and mode of plate tectonics have all been suggested as important contributors to our currently benign environment. The focus for the next 25 years will be to characterize potentially habitable planets, understand how they formed, and search for any tentative atmospheric signs of life.

Phil Armitage leads the planet formation group at the Flatiron Institute’s Center for Computational Astrophysics.

This article is part of the “Five Things We Used to Think Were True” section of the foundation’s 25th anniversary book.