As of January 2018, 6,355 possible exoplanets– extrasolar planets, or planets outside of our solar system- have been detected by preliminary tests. Further tests have confirmed that 3,726 of those candidates are indeed planets. If “other Earths” are defined as planets where we could walk out of a spaceship with no equipment other than a picnic basket, and enjoy a pleasant afternoon on a grassy slope near a stream, then it remains to be seen whether any of these planets fit the description. On the other hand, if “other Earths” refers to rocky worlds approximately Earth’s size, and orbiting within their star’s habitable zone (the zone in which liquid water, and potentially life, can exist), then there is cautious optimism that we have found at least 53 such worlds.

Part of the uncertainty about the 53 possible Earth-like worlds is related to their composition. We don’t yet know their composition; however, it is tempting to conclude that they are rocky because they are similar in size to Earth. Remember the rules of the accretion game: you can only begin to collect gas once you are a certain size, and how much matter you collect depends on how far away from the sun you are. Given how large our gas giant and ice giant planets are compared to Earth, and how far away they are from the sun, we would expect that a planet similar in size to Earth, and a similar distance from its star, should be rocky.

But it isn’t quite as simple as that. We are finding that the rules to the accretion game can result in planetary systems very different from our own. For example, in the planetary systems we have observed, it is common to have planets larger than Earth orbiting closer to their star than Mercury does to the sun. Planets as large as Jupiter are rare, and where large planets do exist, they are much closer to their star than Jupiter is to the sun. To summarize, we need to be cautious about drawing conclusions from our own solar system, just in case we are basing those conclusions on something truly unusual.

On the other hand, the seemingly unique features of our solar system would make planetary systems like ours difficult to spot. One of the ways exoplanets are detected is by measuring the brightness of stars, and looking for tiny variations in brightness that could be caused by a planet passing between the star it orbits and the instrument observing the star. Small planets are harder to detect because they block less of a star’s light than larger planets. Larger planets farther from a star, like our gas giant planets, are difficult to spot because they don’t go past the star as frequently. For example, Jupiter goes around the sun once every 12 years. If someone were observing our solar system, they might have to watch for 12 years to see Jupiter go past the sun once. For Saturn, they might have to watch for 30 years.