Here is what you need to know about how best to get a glimpse of this celestial parade.

- What is happening exactly? -

All eight planets in the solar system orbit the Sun at different speeds but on roughly the same plane.

This is because the planets are thought to have been formed by a massive ring of gas and dust around 4.6 billion years ago -- and they just kept on circling around that track.

"Every so often, when everything works out right, they all appear on the same side" of Earth, which makes them all visible in the sky at once, David Armstrong, an exoplanet researcher at the University of Warwick in the UK, told AFP.

This means that the planets look like a straightish line from our viewpoint on Earth -- but are not actually in a straight line across the solar system.

That can happen but is even more rare, Armstrong said, adding it was of interest in a "science fiction" way than for hard science.

- How can I see this parade? -

"Step outside and look up," Armstrong said simply. The best time to see most of the planets in the Northern Hemisphere will be just after sunset on Friday.

The further north from the equator you live, the better view you are likely to have, he added.

A clear sky will also be key -- no certainty in the Northern Hemisphere's winter -- and being in a dark area away from city lights is a bonus.

But seeing all seven planets will require a telescope or strong binoculars.

- Which planets are easiest to see? -

First look out for Venus, which should be the brightest thing in the sky at twilight, Armstrong said.

Also easy to spot will be the noticeably red Mars and massive Jupiter, both of which are relatively close to the famous Orion constellation.

Mercury will be "just visible where the Sun is setting for a short period of time," said astronomy education officer Jessica Lee, of the Royal Observatory Greenwich in London.

Both Mercury and Venus appear near the horizon because both planets are closer to the Sun than Earth, Armstrong observed.

Yellowy Saturn, which will appear just above the horizon as the Sun is setting, will be more tricky.

"You might be able to see it if you have a clear view of the western horizon," Lee said.

Neptune and Uranus, which sit in the freezing, far-off outer solar system, will require a telescope or high-powered binoculars to get a good look at.

Faint Uranus will be close to Jupiter, Lee said.

If you do have a telescope, Armstrong recommended also searching out Jupiter's Galilean moons.

"They look like little diamonds around the side of the planet," Armstrong said.

Some of Jupiter's moons are thought to have massive oceans hiding under their icy shells, making them prime targets in the search of extra-terrestrial life in our solar system.

- What if there are clouds? -

While the planetary parade technically aligns on the last day of February, many of the planets have already been visible for weeks.

And many including Jupiter, Mars, Venus and Saturn will still be visible in the sky for days to come.

"It's absolutely not like this night or never," Armstrong said.

But for Friday, weather will be key.

"Just hope for a clear night," Armstrong said.