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Thrilling Discovery of Seven Earth-Sized Planets Orbiting Nearby Star (theguardian.com)
At a press conference on Wednesday, NASA scientists announced that they have spotted seven Earth-sized planets orbiting closely around a small, ultra-cool star. The star is 39 light years away. From a report on The Guardian: It is the first time that so many Earth-sized planets have been found in orbit around the same star, an unexpected haul that suggests the Milky Way may be teeming with worlds that, in size and firmness underfoot at least, resemble our own rocky home. The planets closely circle a dwarf star named Trappist-1, which at 39 light years away makes the system a prime candidate to search for signs of life. Only marginally larger than Jupiter, the star shines with a feeble light about 2,000 times fainter than our sun. "The star is so small and cold that the seven planets are temperate, which means that they could have some liquid water and maybe life, by extension, on the surface," said Michael Gillon, an astrophysicist at the University of Liege in Belgium. [...] While the planets have Earth-like dimensions, their sizes ranging from 25 percent smaller to 10 percent larger, they could not be more different in other features. Most striking is how compact the planet's orbits are. Mercury, the innermost planet in the solar system, is six times farther from the sun than the outermost seventh planet is from Trappist-1.
These were found using the transit method, which measures a star's reduction in it's brightness as something passes in front of it.
They know that these sorts of occurrences are not things like sunspots because they can follow up using measurements of the wobbling of the star due to the gravity the planets exert on it.
Combine the two and you have a reasonable inference that there are planets orbiting this star.
If these are not planets, given the above two types of evidence correlating with each other, what else could they be?