Searching for the 'holy grail' of planets

The holy grail in our search for exoplanets (planets orbiting other stars) is to find one with water and at the right distance from its star for that water to be present on its surface as a liquid.

Now it looks as though such a planet may have been found. It is early days yet, but the evidence looks convincing.

Around 100 light years away, in the constellation of Draco, "The Dragon", there are two red dwarf stars orbiting each other. Orbiting one of these stars is a very interesting planet, which has been given the romantic name of TOI-1452 b, which could be just what we have been hoping to find. Draco sits in the northern sky, more or less wrapping closely around the constellation of Ursa Minor, "The Little Bear", which has the Pole or North Star marking the end of its tail.

So this pair of stars and that intriguing planet are high in our skies every night.

The usual way we search for exoplanets is to observe stars for a long time, looking for minute dimmings as their planets pass in front of them. This might sound a bit "dodgy" but it works. Thousands of planets have been discovered, with some of them being found by backyard astronomers.

From the spacings between the dimmings, their durations and how the brightness rises and falls at the beginning and end of each dimming, it is possible to estimate the orbit, the size and the mass of the planet.

The orbit tells us how far it is from its star, which in turn gives us an idea of its surface temperature. This particular planet lies in its star's "Goldilocks Zone", the range of distances giving surface temperatures that would allow liquid water to exist there.

This is an exciting result in itself, but things get more interesting when we look at the size and mass of the planet. It is nearly five times the mass of the Earth, but its mass is too small for its size.

Rocky planets like the Earth, Mars or Venus are basically big lumps of basalt rock with iron cores in the middle.

We can interpret the average density of a planet (mass divided by volume) in terms of different combinations of iron and basalt. However, the density of the new planet is too low to interpret in terms of any combination of these ingredients.

It needs a third ingredient, one that is a lot less dense than basalt. That ingredient is likely to be water. Basalt has a density of about 2.7 gm/cc (grams per cubic centimetre). Water's density is 1 gm/cc. Even so, to account for the density of that new planet, it has to be about 30% water. That is an amazing amount of water.

Our planet looks blue from space because 70% of its surface is covered by water. However, that layer is pretty thin compared with the diameter of the planet. Our world is only about 1% water. The rest is the usual iron and basalt mixture. Ours is quite a dry planet.

That huge relative amount of water suggests TOI-1452 b is covered by an ocean that is many kilometres deep everywhere, not just in occasional deep spots or trenches as we have here.

Europa and Enceladus, moons in our Solar System have deep oceans, but in their cases they are tidally heated and covered by enormously thick layers of ice.

On this new world, the ocean would be open to the sky, and illuminated by the planet's red sun. This means, just as in our oceans, photosynthesis could be possible, which means seaweeds and phytoplankton could be floating around. However, at this point, the low density might have another explanation, namely that it is a sort of mini-Jupiter, with a rocky core surrounded by a dense, opaque atmosphere, so we are estimating the size of the atmosphere, not the size of the planet.

More work is needed to see which of these possibilities is the case. On the other hand, this discovery is one of the most exciting ones yet.

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• Saturn and Jupiter are in the sky after sunset. Mars rises three hours later followed, just as the sky starts to brighten for dawn, by Venus.

• The Moon will be full on Sept.10.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.