Our planet’s demise not a surprise

Imagine a planet slowly spiralling in towards its star. As it gets closer the increasing heat, together with the star’s equivalent of the solar wind, blast away the atmosphere.

Then, as what is left of the planet continues its inward spiral, it disintegrates into a shower of falling fragments.

For the first time, astronomers have actually observed this happening. The star in question, poetically named ZTF SLRN2020, lies in the constellation of Aquila, “The Eagle”, at a distance of some 12,000 light years. It might be a sobering thought that the Earth will share a similar fate in a few billion years.

Imagine a system of planets orbiting another star. For most of its life, the star, like our sun, behaves itself, shining reasonably steadily, and keeping any lifebearing planets comfortable. Then, like all stars, it starts to run low on fuel. This, paradoxically, causes it to expand into a red giant, and to radiate more energy.

Planets that orbit close in find themselves having to shoulder their way through the gas and dust making up the star’s extended envelope, instead of moving more or less frictionlessly through a near vacuum. The drag sucks energy from the planet and its orbit turns into an inward spiral. Ploughing through the gas and dust, together with the increasing heat from the star, gradually strips away the planet’s atmosphere and boils away any oceans it might have had, turning it into a dead world. Intriguingly though, the story does not end there.

When we swing a weight on a piece of string in circles around our heads, we feel an outward pull on the string. This is actually the inertia of the weight resisting being pulled into a curved path rather than doing what it really wants to, which is fly off in a straight line. We have come to refer to this outward pull, not completely accurately, as “centrifugal force.” This “force” is related to two things: the speed the object is moving and the diameter of the circle in which it moves. The smaller the circle, or the higher the speed, the stronger the force.

A planet orbiting a star in a more or less circular orbit is moving so that the inward-directed gravitational pull of the star is balanced by the outwarddirected centrifugal force.

Half the planet is closer to the star than the centre of gravity. The other side is further away. Since the planet is a solid object, all parts of it are moving at the same speed.

Therefore the outer part of the planet is moving faster than is needed, and the centrifugal force is bigger than the pull of the star’s gravity, pulling that part of the planet outwards.

Similarly, the half of the planet closest to the star is not moving fast enough, and there is a net inward force due to gravity being stronger than the centrifugal force.

The result is the part of the planet closest to the star is being pulled inward, and the outer part pulled outwards. This stretching force is often referred to as a tidal force, because it is the reason we have ocean tides here on earth.

Normally, as we can see with the planets in the solar system, this tidal force is too small to endanger the planet. However, the gravitational attraction rises rapidly as we get closer to the star, quadrupling each time we halve the distance. The result is that as our ill-fated planet spirals in closer and closer to its star, the tidal force increases rapidly.

Rock is really good at resisting compression, which is why we make pyramids out of it. However, it is not very good at handling stretching.

Eventually, for our planet, the tidal forces become too great and the planet disintegrates, with its fragments falling down into the star. This will probably be the ultimate fate of our planet, although not for billions of years.

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Venus shines brightly in the west after sunset. Mars, much less bright, and reddish, lies a little higher. Saturn, golden coloured and moderately bright, lies low in the dawn glow.