On December 14, NASA and Google announced the discovery of an eighth planet orbiting the star Kepler 90. Other media outlets picked up the hype from there, with teasers like “first solar system like ours” and “boosting the chances of finding alien life elsewhere in the universe.” What’s the stir about?
The Extrasolar Planets Catalog lists 623 multiple planet systems discovered so far.1 Until yesterday, none included as many planets as our solar system. The discovery was made by a machine learning method developed by Google that scoured the Kepler spacecraft storage bank of data.
The number of planets orbiting Kepler 90 is where all similarity with the solar system ends. For starters, the star Kepler 90 is not like the Sun. Its diameter and mass are both about 20 percent larger than the Sun. It is about twice as bright as the Sun. At less than half the Sun’s age, it manifests considerably more intense and frequent flares. The characteristic features of Kepler 90 rule out the possibility of life on all eight of its known planets, the inner seven of which orbit Kepler 90 much closer than Earth orbits the Sun, while the most distant planet orbits Kepler 90 at the same distance Earth orbits the Sun.
The featured image compares the sizes of Kepler 90’s planets to the sizes of the Sun’s planets. All of Kepler 90’s planets are larger than Earth. The nearest three to Kepler 90 possess diameters that are 1.31, 1.18, and 1.32 times larger than Earth’s, respectively. The next three farther out planets possess diameters equal to 2.88, 2.67, and 2.89 times Earth’s, respectively. The two most distantly orbiting planets have diameters equal to 8.18 and 11.32 times Earth’s, respectively. Unlike the solar system, the planets in the Kepler 90 system consistently trend larger with orbital distance.
The biggest difference between Kepler 90’s and the Sun’s planets is their orbital spacing. Kepler 90’s eight planets are jammed tightly together while the Sun’s planets are spread apart from one another. Kepler 90’s planets orbit Kepler 90 at 7.4, 8.9, 12.3, 32, 42, 48, 71, and 101 percent of Earth’s orbital distance from the Sun, respectively. For comparison, the Sun’s eight planets orbit the Sun at 38, 72, 100, 152, 529, 958, 1,914, and 3,120 percent of Earth’ orbital distance, respectively.
Ever since the first planets orbiting hydrogen-burning stars were discovered in 1995, many astronomers have predicted that the population of extrasolar planets would include a large number of planets just like the planets in the solar system. None of the planets in the Kepler 90 system even comes close to matching the characteristics of any one of the Sun’s planets. In fact, after more than 20 years of planet hunting, none of the 3,720 planets in the Extrasolar Planets Encylopedia and Catalog matches any of the features of the Sun’s planets.
The extrasolar planet that comes the closest to matching a solar system planet is upsilon Andromedae e. That planet has nearly the same mass, orbital distance, and orbital shape as does Jupiter. However, upsilon Andromedae e lacks one of Jupiter’s features that is critical for making advanced life in the same system possible. Jupiter is the most massive planet in the solar system. It possesses 68 percent of the total mass of the Sun’s planets.
Upsilon Andromedae e is dwarfed by planets upsilon Andromedae c and d, which possess masses that are at least 14.0 and 10.3 times more massive than Jupiter. Both planets also orbit upsilon Andromedae much closer than does upsilon Andromedae e. Both planets by themselves eliminate any possibility of another planet in the upsilon Andromedae system being able to sustain life.
Given how densely packed Kepler 90’s planets are, astronomers are persuaded that Kepler 90 probably possesses more than just the eight planets discovered so far. The announcement by NASA and Google adds to the growing weight of evidence that the solar system is not normative. It appears to be unique. All the evidence shouts that the solar system is special in its exquisite designs that enable it to be a home for advanced life.
- Exoplanet TEAM, The Extrasolar Planets Encyclopedia, Catalog (December 14, 2017): https://exoplanet.eu/catalog/.