Physicist Dr Nicholas Rattenbury explains what today's 'Proxima b' planetary discovery means

The planet - called Proxima b for now - orbits in the region around its host star (above) at a distance astronomers think would allow liquid water to exist on its surface. Picture / Wikipedia

In today's big science story, researchers have discovered what they believe to be a new planet, the closest one ever detected outside our solar system.
It is a small, rocky planet not unlike our own, orbiting the sun's closest stellar neighbour. Auckland University physicist Dr Nicholas Rattenbury shares his thoughts on it.

The discovery is of a signal consistent with a planet orbiting the nearest star to our Sun, Proxima Centauri.

The planet - which is called, for now, Proxima b - orbits in the region around its host star at a distance which we think would allow liquid water to exist on its surface, given the amount of radiation received at the planet's surface from its host star.

The analysis of the signal suggests that the planet could have a mass as low as 1.3 times the mass of the Earth.

These two findings are exciting, as we are interested in finding planets which could host liquid surface water, as we think this is a very important ingredient for a planet to harbour life.

Apart from being at the right distance from its host star, to have the sort of conditions suitable for surface life such as that we find on Earth, we think that a planet must have an atmosphere and the nature of that atmosphere is vital.

This, in turn, means that the planet mustn't be too large or two small.

Too large and it would most likely have a thick, dense atmosphere, like a Neptune or a Jupiter, and we find it harder to envisage life like that we find on our Earth on such gaseous planets.

Similarly, the planet can't be too small, or it won't have enough mass to hold onto an atmosphere for long enough for life to emerge.

Mars in our solar system is smaller than Earth and lost its atmosphere over time, making it impossible for liquid water to remain on the surface.

The newly discovered planet, Proxima b, might have a mass in the right range so that it too would have just the right amount of atmosphere to keep the surface warm and under enough pressure to keep water liquid on its surface.

However, there hasn't been any detection of an atmosphere on Proxima b or of what gases that atmosphere might consist.

No doubt the Proxima Centuri planetary system will now be the subject of further investigation, using other techniques to confirm the existence of the planet.

Ideally we'd like to see transit observations of the planet - to see if Proxima b passes between us and its host star Proxima Centauri.

This would cause a dip in the amount of light we see coming from Proxima Centauri, and would give us the radius of the planet.

More importantly, such transit observations would get rid of that pesky word "minimum" in front of the phrase in the paper that the planet has a "minimum mass of about 1.3 Earth masses".

As it stands, the planet could be even more massive than Jupiter - and correspondingly be a planet very different to Earth - we simply don't know yet. Further observations will hopefully resolve this issue.

For now, it is an exciting discovery, mainly because we have found evidence of a planet orbiting the star nearest to our own sun.

The discovery is consistent with results from the exoplanet community which suggests that the galaxy has planets in abundance, so perhaps we shouldn't be surprised that the star nearest to our sun also hosts at least one planet.

To say that I'm not surprised isn't to say that I'm not excited.

If we choose to design a space probe mission to explore other planetary systems, then the Proxima Centauri system no doubt will be top of the list of systems to investigate.