Oh dear, here we go again. The world’s media are falling over themselves to report, for what seems like the two millionth time, the discovery of the “first Earth-like planet around another star”. This one is called Kepler-22b, has been discovered by NASA’s Kepler mission, and lies in the so-called “habitable zone” of its parent star, ie the region where conditions are just right for life as we know it to exist. See the
NASA press release and an example of hyperbolic media nonsense on the BBC website here .
This isn’t the first time we’ve heard exaggerated claims of the discovery of the “first Earth-like planet in habitable zone around another star”. Check out this story from 2010 for example.
So what’s wrong with this announcement?
Well, first of all, Kepler-22b is significantly larger than Earth. Indeed, its radius is 2.4x bigger than ours. And we don’t know its mass. Unfortunately, as with a lot of Kepler’s candidate planets, it orbits a star that is too faint for us to measure the planet’s mass by the radial velocity method. If we assume it has the same density as Earth, then it has a mass around 13x that of Earth.
For comparison, Uranus has a mass 14.5x Earth’s, and a radius about 4x Earth’s.
So in truth, we have no idea whether Kepler-22b is rocky, or a gas planet like Uranus.
Then there’s the claim that the planet’s temperature is 22 degrees centigrade. That’s right, as precise a measurement as 22 degrees. Pleeeeeze. Meteorologists often cannot agree on the temperature in their own back gardens. It depends on whether you place the thermometer in the shade or in the Sun, for a start.
Kepler-22b is clearly an interesting discovery, and I don’t have any quibble with the claim that it likely lies in its host star’s “habitable zone”. But it is tiresome, and in the long-term potentially damaging to astronomy’s credibility, to exaggerate the claim that it is “Earth-like”, when that can simply be shown not to be true.
One day, someone (quite possibly Kepler) will find a bona fide one Earth mass, one Earth radius rocky planet orbiting a Sun-like star at just about the same distance we do. Let’s hope we haven’t Cried Wolf once too often before then…..