The newly found planet (centre) is about the size of Mars (left) and just over half as big as Earth (right) (Image: NASA/JPL-Caltch)

The smallest exoplanet yet found around a sun-like star is a rocky world half the size of Earth and almost identical in size to Mars. Although it is too hot for life, researchers say its discovery boosts the chances of finding other, more life-friendly planets.

The newly discovered planet, called KOI-961.03, periodically passes in front of its parent star, causing a slight dip in its brightness detected by NASA’s Kepler space telescope.

Philip Muirhead of the California Institute of Technology in Pasadena led a team that made the discovery, which was announced on Wednesday at a meeting of the American Astronomical Society in Austin, Texas.


The planet’s parent star is a red dwarf, puny in size compared to the sun but the most common type of star in the galaxy. Two other newly discovered rocky planets, KOI-961.01 and KOI-961.02, also orbit the star.

The researchers used the Keck Observatory in Hawaii to help pin down the size of the host star. That allowed them to calculate the planets’ sizes based on the amount of dimming they caused.

KOI-961.03 is the smallest at 0.57 times the size of Earth. That makes it just 8 per cent larger than Mars. The other two planets, KOI-961.02 and KOI-961.01, are 0.73 and 0.78 times the size of Earth, respectively.

Smaller and smaller

The smallest and second-smallest planets in the system are tinier than any previously discovered exoplanet around a normal living star (one smaller planet – just 2 per cent as massive as Earth – was previously found around a dead star called a pulsar).

The record for the smallest planet known has been broken repeatedly in recent weeks. On 20 December, two planets – just 0.87 and 1.03 times the size of Earth – were announced. The very next day, an even smaller planet 0.76 times the size of Earth was announced, along with another planet 0.87 times the size of Earth. All four were discovered with Kepler.

The newly discovered system is the most compact yet found, with all three planets orbiting the star at less than 1 per cent of the Earth’s distance from the sun and taking less than 2 days to complete an orbit.

The Mars-sized planet is the farthest of the three planets from the star but is still far too hot at an estimated 200 °C to harbour liquid water and life as we know it.

‘Like cockroaches’

The discovery adds to the growing evidence that small, rocky planets are common and suggests that such planets are also common around red dwarfs, which make up 80 per cent of the stars in the galaxy.

Only a few dozen of the 150,000 stars monitored by Kepler are as small as KOI-961, the parent star of the new planets. And because Kepler can only see planets that pass in front of their parent stars, most are invisible to it.

To have detected rocky planets around one of them suggests that at least one-third of the smallest red dwarfs possess such companions, said team member John Johnson of Caltech in Pasadena. “It’s kind of like cockroaches – if you see one, then there’s dozens hiding,” he said in a press briefing at the AAS meeting.

Many of planets orbiting red dwarfs probably lie in their stars’ habitable zones, where liquid water can survive on an object’s surface, Johnson said: “The whole galaxy must be just swarming with little habitable planets around faint red dwarfs.”