No event is free of controversy these days - not even the discovery of seven habitable planets that hit the news late last month. For a few days, drawings of those tranquil spheres loomed above the tumult of earthly affairs - the Trump tweets, the protests, the botched Oscars. But then a friend, who happens to be trained as a biologist, wrote to me complaining that by declaring these worlds "habitable," NASA's PR people were promoting a space-exploration delusion.

He has a point. Like many people who follow astronomy, I'd started to take for granted that when astronomers say the word "habitable" they don't actually mean habitable - not in the sense most people use the word, to describe a place where you can go and not die instantly.

Scientists can't actually see any of these planets (pretty pictures like this are an artist's renditions). Credit:NASA/JPL

This is not to detract from the value of the discovery of seven earth-sized worlds orbiting another star. At a distance of 40 light years, it is in our galactic neighbourhood, so scientists have a shot at detecting atmospheres around the planets. The finding adds to a growing understanding that our galaxy is bursting with planets - astronomers estimate that most of the 100 billion stars in the Milky Way have at least one. But for all practical purposes, they are bad real estate.

Take this new trove. One big problem is that they orbit a dim sun - a dwarf star called Trappist-1, which is about one-twelfth the size of our sun and much cooler. The planets all orbit cozied up to the star, which keeps them from freezing over, but getting a front row seat in your solar system is not a great deal. Such planets get blasted with high-energy radiation, and the gravitational pull of the star probably keeps them from rotating. That means one side gets perpetual sun exposure and the other perpetual darkness. Day, night, morning and evening would be places, not times of day.