Nearly all of the planets in the solar system are about to march through the night sky in a single-file line. This planetary alignment, sometimes called a planet parade, will include all of the solar system’s planets except Mars, as it is currently on the opposite side of the sun from Earth and therefore not visible.

Alignments like this only occur every few years, when all the planets’ orbits happen to carry them to the same side of the sun at the same time. All of their orbits are different lengths – Mercury takes 88 Earth days to circle the sun while Neptune takes about 165 Earth years – so planetary alignments are a lucky coincidence of geometry and orbital dynamics.

Sometimes they happen relatively close together – February 2025 saw a so-called “great alignment”, where all seven planets were visible at once – and sometimes years go by without a single one.

During a planetary alignment, the planets trace a line across the sky along what’s called the ecliptic. This is the same line that the sun follows across the sky during the day, although the tilts of the planets’ orbits make it so that they don’t line up perfectly. From beyond the solar system, the planets would not appear in a line – that is an optical illusion due to the fact that all of the planets orbit in the same plane.

The alignment will be visible across a range of dates depending on where you are in the world, but 28 February and 1 March will be the best days to see it in most places. Find a spot with a clear view of the western sky and as little light pollution as possible and look out for the parade.

The best time to see the planet parade on 28 February will be less than an hour or so after sunset: Mercury’s orbit close to the sun means that it will sink below the horizon shortly after the sun does. Right after sunset, both Mercury and Venus will be visible low on the western horizon. Saturn and Neptune will be just above them, then Uranus, and finally Jupiter relatively close to the nearly-full moon.

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While Mercury, Venus, Saturn and Jupiter should be visible to the naked eye, binoculars will be necessary to spot Uranus, and a telescope to see Neptune, because they are so far away.

Astronomers have found a planetary system that seems to have formed inside-out. While most systems, like our own, have rocky planets closest to their star and gaseous ones further out, the LHS 1903 system has a rocky world at its edge, challenging established models of planet formation.

The outermost of the system’s four planets wasn’t immediately apparent in initial observations from the Transiting Exoplanet Survey Satellite – those first measurements allowed researchers to identify one rocky planet a little bigger than Earth close to the star, plus two gaseous ones slightly smaller than Neptune beyond that. But when at McMaster University in Hamilton, Canada, and his colleagues followed up on the system using eight other observatories, they spotted the telltale signatures of a fourth world that is marginally bigger than the other rocky planet in the system.

This rocky world, which is further from the star than its gaseous siblings, was unexpected. “These systems are not unheard-of, but they’re rare – and the systems that have this unique architecture, and for which we can characterise them in detail, are extraordinarily rare,” says Cloutier.

Those details, including the sizes of the planets and the fact that they all orbit their star in periods of less than 30 Earth days, made it possible for the researchers to test models of how these planets may have formed. “Producing one planet can be done with several mechanisms, but once you need to produce four different ones, you can start to discriminate between different models,” says at Leiden University in the Netherlands. “You have find a model that can explain all of them.”

Most systems are thought to form all of their planets at about the same time from the same disc of dust and gas. The sizes and compositions of the planets are dependent on where they formed within that disc and what events, such as collisions with other worlds, happened to them afterwards. For the LHS 1903 system, though, that model doesn’t work.

If the planets of LHS 1903 were born in the traditional way, the outermost one should have formed with a thick gaseous envelope like the middle two. That atmosphere could have been lost through a collision or bombardment with radiation, but the researchers’ simulations show that such a process would have also stripped away the gas from one or both of the inner planets.

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“It’s really difficult for you to sculpt the outermost planet without affecting those gaseous planets that are closer to the star,” says Cloutier. But the orbital dynamics of the system make it extraordinarily unlikely that any of the planets wasn’t born from the same disc.

Cloutier and his team found that the most likely way for this system to be created is through a process called “inside-out” planet formation. Here, a single planet forms and then migrates inwards towards the star, making room for the next planet, and so on. This takes time, so the planets are born in different environments as the protoplanetary disc evolves. “That final planet, if it’s taken long enough, it has formed in an environment where there’s no gas available,” says Cloutier. This system goes to show how diverse the planetary formation processes in the universe might be, he says.

Mysteries of the universe: Cheshire, England

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Around the nearby star Fomalhaut, asteroids are smashing into each other in a series of cosmic cataclysms, creating huge clouds of dust. For the first time, astronomers are watching one of these collisions as it occurs, which could provide a window into the early days of our own solar system.

Fomalhaut has a history of strange observations: in 2008, at the University of California, Berkeley, and his colleagues reported what seemed to be a giant planet in orbit around the young star, based on observations with the Hubble Space Telescope made in 2004 and 2005. Over the years, though, as more observations have rolled in, researchers have hotly debated over what this strange object, called Fomalhaut b, might be. It was either a planet a bit larger than Jupiter, or a cloud of debris.

Now, Kalas and his team have used Hubble to look at Fomalhaut once again. “In 2023, we used the same instrument we’d used [before], and we did not detect Fomalhaut b – it wasn’t visible anymore,” says Kalas. “But what really shocked us was [that] there was a new Fomalhaut b.”

This new bright spot, called Fomalhaut cs2 (short for “circumstellar source”), couldn’t be a planet, or it would have been seen sooner. The best explanation is that it is a cloud of dust created by the collision of two large asteroids, or planetesimals, each around 60 kilometres in diameter. The disappearance of Fomalhaut b hints that it was probably a similar dust cloud all along.

“These sources are noisy and erratic, so we’re still some ways off a firm conclusion,” says at Columbia University. “But, all of the evidence to date seems to fit neatly under the umbrella explanation of collisions between proto-planets in a nascent system.”

Spotting two such smash-ups is unexpected, though. “Theory dictates that you shouldn’t see these collisions except once every 100,000 years or rarer. And yet, for some reason, we’ve seen 2 events in 20 years,” says Kalas. “Fomalhaut is sparkling like a holiday tree, and that is a surprise.”

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It may mean that collisions between planetesimals are more common than we had thought, at least around relatively young stars like Fomalhaut. Kalas and his colleagues have more observations scheduled over the next three years with both Hubble and the more powerful James Webb Space Telescope (JWST) to watch how Fomalhaut cs2 behaves moving forward and to try to find the now-dimmer Fomalhaut b.

This is a unique opportunity to study these collisions in real time. “We no longer have to depend solely on theory to understand these violent impacts; we can actually see them,” says Kalas. More observations could teach us not just about young planetary systems in general, but also about our own early solar system and where it fits in the cosmic menagerie.

“We’ve long wondered if the moon-forming impact was typical or not beyond our cosmic shore, and here we see compelling evidence that collisions are par for the course,” says Kipping. “Perhaps we’re not as unusual as some have speculated.”

Mysteries of the universe: Cheshire, England

Spend a weekend with some of the brightest minds in science, as you explore the mysteries of the universe in an exciting programme that includes an excursion to see the iconic Lovell Telescope.

Find out more