NASA has released a new catalogue of exoplanets discovered by the Kepler mission. The list contains a total of 4,034 exoplanet discoveries, including 219 new finds.

Of these 219 new exoplanets there are 10 exoplanets that have a size nearly equal to that of our planet Earth and are said to be residing in the Goldilocks zone aka habitable zone – a a zone that would permit liquid water on the surface of an Earth-like world. These 10 new exoplanets similar to Earth’s size are on top of the 40 other we already known about. However, being similar in size to Earth and to be located in the Goldilocks zone isn’t enough for a planet to harbor life.

If we look at our Solar System, planets can be classified into either terrestrial planets – ones with rocky surfaces like Earth – or gas giants like Jupiter, whose solid cores are buried beneath thousands of kilometres of enveloping gases. However, this classification doesn’t stand at all when it comes to exoplanets discovered by Kepler Mission.

There are exoplanets that are even closer to their star than Mercury and still are gas giants, while there are those far away worlds from their stars that have hot surfaces. There are those planets that orbit around binary stars like Luke Skywalker’s fictional home world of Tatooine in the film franchise Star Wars. But perhaps the most mysterious entries are the so-called super Earths.

What are these super Earths? Over two-thirds of the planets in the Kepler Mission Catalogue have radii of between 1.1 and four Earths, placing them between Earth and Neptune in size. These super Earths are the most common planet discovered, yet our own Solar System has no analogue to tell us what such worlds might be like.

Are they terrestrial planets with rocky surfaces or mini gas giants with crushing atmospheric pressures? As interest has mounted in the prospect of finding a habitable planet, a key question is whether any of these worlds have a solid surface.

At the media briefing, NASA revealed that the Kepler Mission catalogue had produced a clue. A recent journal paper had spotted a split in the distribution of super-Earth planet sizes.

While planets with radii of around 1.3 Earths and 2.4 Earths are equally common, planets with sizes between 1.5 and two Earth radii are much harder to find. The astronomers speculate that this is the division between giant rocky planets and gaseous mini-Neptunes.

Although this strong divide in the super-Earth population has not previously been observed, its location agrees with the small number of planets we have bulk density measurements for. These density values suggest that planets larger than about 1.6 Earth radii have thick, Neptune-like atmospheres.

One possibility for the split in the super-Earth population is that the rocky planets have had their atmospheres stripped by radiation pouring from a star. Alternatively, the rocky worlds may have formed later from the gas-rich planet-forming disc that dispersed before the planets could accrue a thick atmosphere.

We meet a similar problem for the 50 planets found within the habitable zone. Knowing only their size and location is not enough to determine if their surfaces are similar to the Earth.

A difference in geology could create a very different atmosphere to our own, or the inability to create protective magnetic fields to shield the planet from sterilising stellar flares. The temperature may even be perfect for liquid water, but the planet entirely dry.