This movie shows how someone standing on a hypothetical moon orbiting the planet would see three stars and then the planet itself moving through the sky. Credit: © NASA/JPL-Caltech

Meet the impossible planet. This world nestles inside a system containing three stars that, according to current theories, should have denied it the chance to develop.

But Maciej Konacki, an astronomer at the California Institute of Technology, Pasadena, has been surveying complicated star systems to prove these theories wrong.

Konacki's planet is in the triple-star system known as HD 188753, which lies about 149 light years from Earth, in the Cygnus constellation. The star at the centre of the system is very much like our own Sun. Its planet, which is at least 14% larger than Jupiter, orbits the star once every 80 hours or so, at a distance of about 8 million kilometres, a twentieth of the distance between Earth and the Sun.

You shouldn't see it, but you do see it. Artie Hatzes , Thuringia State Observatory in Germany

Two more stars, whirling tightly around each other, orbit the central sun at a distance that would put them between Saturn and Uranus in our own Solar System. Konacki identified the planet by watching the way in which the three stars' orbits are affected by its gravity, using the Keck I telescope in Hawaii.

The planet would be a very strange place to visit. "With three suns, the sky view must be out of this world," says Konacki, who likens it to the vista seen by Luke Skywalker in Star Wars as he watches two suns set from his home planet of Tatooine. And the discovery, reported in this week's Nature1, will certainly put planetary formation theories under pressure.

Icy seeds

Planets are thought to form from the dusty disks of material that surround young stars. Icy nuggets in the disk act as seeds that slowly accumulate enough dirt to build up into a planet.

But many of the 161 candidate planets so far spotted outside our Solar System are 'hot Jupiters'. These are similar to our own system's giant planet but orbit extremely close to their sun. They could not have formed in the orbits they currently occupy, astronomers argue, because such regions would always have been too hot for an icy core to exist. Instead, theorists suggest that the planets must form in a colder area far from their stars, and then migrate inwards due to drag from remaining material swirling around the systems' centres.

The trouble is that this could not have happened in HD 188753, says Konacki. The outer pair of stars would have beaten most planet-forming material into oblivion, leaving nothing in the cold region that could form an ice core. The remaining dust would not have extended much beyond the hotter region that spans a distance equal to that between the Earth and the Sun.

This makes HD 188753 something of an enigma. "You shouldn't see it, but you do see it," says Artie Hatzes, an astronomer at the Thuringia State Observatory in Germany.

Nature finds a way

Hatzes thinks the most likely answer is that the planet formed in its present orbit, without an icy core. "Maybe nature found a way," he says. One possibility is that the core could have formed from less volatile materials, such as tarry hydrocarbons.

Konacki suggests instead that hot Jupiters could be failed stars. So in HD 188753, the planet at the centre of the three-star system could be a fourth potential star that just had insufficient matter to start fusing hydrogen into helium, leaving a gas giant close to the central sun.

Both Hatzes and Konacki agree that more of these unusual systems will be found. "Most of the stars in our galaxy are in multiple systems; our own Sun is in a minority," says Hatzes. Konacki is now continuing with his survey of 450 multiple-star systems to look for more surprising planets.