how can we differentiate them from stars and comets?

This Tuesday, January 21, 2025, six planets will be “aligned” in the sky. Concretely, they will not be exactly on the same line, but we will indeed be able to see them at the same time. We are talking about alignment from three planets, and this is not uncommon. Six at once, on the other hand, is a little more.

In other words, the entire solar system, minus Mercury (and not counting the Earth which we can hardly see from above), will be visible in a single observation.

A good evening to sit outside (despite the cold), and dream a little with your eyes up. But how can we be sure that we are really observing Mars, and not just any star?

Venus, one of the brightest points in the sky

To the naked eye, the task seems difficult, and the seven other planets of the solar system (Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune) can very quickly be confused with Aldebaran, Vega, or even with this random comet who walks in the cosmos.

In fact, you have to keep in mind that not all white dots you see in the night sky are the same.

There are the stars therefore, which, very roughly, are the suns of other solar systems (the detailed explanation here); the planets, which revolve around our Sun (our star, therefore); and a host of stellar objects, ranging from comets to asteroids, including artificial satellites such as Starlink or the ISS.

At such distances, it is very difficult to differentiate the comet from the star from the planet. Everything is just a white spot, especially with our vision. All ? No.

The tip to know

Actually, there are two tricks. Already, if you concentrate carefully, the planets seem larger and more flattened. It's simply because they are closer.

Then, still due to their distance, the stars twinkle. They are so far from us that their light must travel an immense distance before reaching us. And who says long distance means higher probability of being disrupted.

It is therefore a simple physical reaction: the stars twinkle due to the refraction effect of the earth's atmosphere, which their light passes through before reaching our eyes.

A phenomenon which does not or very little affects the planets which are much closer to us. Neptune, the most distant planet in our system, is approximately 4.3 billion km away. Conversely, Proxima Centauri, the closest star to us, is 4.24 light years away, or 40,113 billion kilometers.

Be careful, the stars don't blink either: it takes time and a little experience to be able to make the distinction. “If the weather is clear, it is clear,” wants to reassure Gilles Dawidowicz, vice-president of the Astronomical Society of France, contacted by actu.fr.

If the infographic is not displaying correctly, click here.

If it really flashes, it's definitely a plane.

How not to miss anything of the show?

But, this tip can still be used during long evenings of space observation, and more particularly this Tuesday, January 21, 2025, during the alignment of the planets.

As a reminder, you will have to wait for the Sun to set, and look towards the south-southwest. To be sure of your success, don't hesitate to download an application like StarWalk or Stellarium.

Even on February 28, when, rare, the seven planets will all be visible in the same plane.

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