Discovered by NASA’s new TESS (Transiting Exoplanets Survey Satellite) mission, the new alien planet ‘sits just outside our solar system’.
HD 21749b is three times the diameter of Earth and 23 times the mass, so it’s more of a midsize planet than a small one.
Planets of that size are not unusual, but the size and inferred density of HD 21749b place it right on the dividing line between rocky, Earthlike planets and slightly bigger, gassy worlds that resemble Neptune.
Scientists are eager to understand why planets seem to fall strongly into one category or the other.
There’s nothing particularly strange about the temperature of HD 21749b, either. Its “equilibrium temperature” is about 300 degrees F (and it could be warmer still if it has a heat-trapping atmosphere), so it’s not exactly cool by human standards. But it is relatively cool compared to many other known super-Earth and mini-Neptune planets. There is no equivalent planet in our solar system, and we don’t know much about what conditions might be like on such a world. Now we can find out!
HD 21749b happens to be fairly nearby (53 light years away), and it transits its star, meaning that it appears to pass right in front of the star and cast a little shadow as seen from Earth. Watching how the star’s light passes through the planet’s atmosphere can reveal the composition of the planet, the layering of its atmosphere, and even the presence of clouds. Finding nearby transiting planets is the purpose of TESS.