Gee (J357d) ... what a planet

THE number of known extrasolar planets — that is, planets orbiting stars other than our Sun — reached 4103 this week.

This number will certainly increase, because astronomers will continue to pursue this fascinating work.

One of the tantalising aspects of this study is finding planets that have similar properties to ours, and have a possibility of harbouring life.

One of this year’s discoveries — a planet called GJ357d orbiting a red dwarf star — falls into this category. It has become one of the most important planets for further study with the highly advanced telescopes that will come into use in the near future.

There are several methods that astronomers use to detect extrasolar planets, but it is rare for one to be observable directly — because of the glare of the parent star.

In the case of the star GJ357, one planet was detected by the Transiting Exoplanet Survey Satellite (TESS), which measured the tiny drop in starlight as the planet moved in front of its star as a result of its orbital motion. This is an event called a transit.

Since then, two more planets, one of which is GJ357d, have been detected using another technique: measuring the “wobble” of the star caused by the gravity of the planets.

As yet, we have still to discover whether GJ357d will also transit across the star. If it does, that will provide more information about it.

However, we already know that it completes each orbit every 55.6 days, and it has a minimum mass of about six times that of Earth. If it is a rocky planet like Earth and this is its actual mass, it would have a diameter about 75 per cent greater than Earth’s.

The most important thing, however, is that it orbits within the so-called habitable zone around the star: a range of orbital distances over which it is possible for the planet to have liquid water.

This does not mean that such a planet is actually habitable, because that depends on several other factors.

Based on our own knowledge of life, though, the possibility that water could exist in liquid form does at least indicate an important place for further study. In combination with this knowledge is the fact that the planet is far enough from the star for it to eventually be imaged and studied separately.

We may be able to learn about the makeup of its atmosphere, which could help to resolve the question of whether life may exist there.

This is because life on a celestial body would alter the composition of the atmosphere in a way that we may be able to measure.

It would not be an actual detection of life itself, but it would be an exciting measurement!

One advantage of being able to detect a transit of this planet across the face of the star GJ357 is that it would pin down the diameter of the planet.

Knowing both the mass and the diameter will immediately tell astronomers its average density — that is, how much material there is, on the average, in a given volume.

For example, if it’s light for its size, it would have a greater amount of ice. (Ice is less dense than rocky material.)

As you can see, some quite basic principles help to explain what the astronomers are doing. So if you are still at school and you wonder why you need to know how to find the volume of a sphere, understand what is meant by density, or are puzzled as to why you need to learn algebra, it’s worth remembering that these types of things are the foundations of good science.

It’s a good time to be focusing on that, as today marks the beginning of Science Week. There are plenty of activities around Australia, and this of course includes Tasmania. Check out the program at scienceweek.net.au.

Martin George is manager of the Launceston Planetarium.