Last April, a celestial alignment surprised NASA, giving them the opportunity to observe Uranus more closely, something they had not done for decades. And it is that, in this alignment, the planet was perfectly positioned between Earth and a star located more than 400 light-years away, a very rare event known as “stellar occultation”,but which allowed NASA to observe this mysterious giant.

Celestial alignment, what is it?

This event happens when several planets, the Moon or the Sun appear to be in a straight line (but only from Earth’s perspective). That is, although in space they are located in three dimensions, from our planet it looks like a straight line of planets.

These events can include planetary conjunctions, alignments of the Moon with planets or stars, or even with the Sun, as happens in eclipses.

And a stellar occultation?

This happens when a celestial object (such as a planet, the Moon or an asteroid) passes in front of a star from our perspective on Earth, blocking its light, but it helps astronomers to study how the light of the object that is in the middle of this occultation behaves, it is more or less like an eclipse, but it allows researchers to obtain information about the composition, rings and its orbit.

How long did this occultation last?

Like eclipses, they are not events that last long, this one, specifically, lasted one hour. The human eye was barely able to distinguish that a star disappeared behind another but, for astronomers, it was something like a Champions League final for soccer.

At this moment, they created a curve with which they could analyse the different layers of Uranus’s atmosphere, they measured temperature, pressure and density, something that had not been done for more than three decades since Voyager 2 in 1986.

If you want to know more about this occultation process, we leave you here a visual representation, you just have to click!

The world looking at Uranus

This event does not happen often, moreover, it was only visible from North America, so NASA Langley scientists joined an international team of 30 astronomers who used 18 professional observatories, a milestone in astronomy. The research was led by William Saunders but it was the first time they collaborated on that scale for an occultation. And it is that, knowing the location of Uranus is not as easy as it seems, it had been almost 30 years since anyone had looked at Uranus (since the last stellar occultation occurred in 1996).

Even NASA’s Infrared Telescope (IRTF), on the summit of Mauna Kea in Hawaii, joined the observation, under the supervision of Emma Dahl, a postdoctoral researcher at Caltech. She herself explained that these events are key because, as they do not have a solid surface, gas and ice giants like Uranus allow studying atmospheric phenomena more cleanly, without interferences.

A general rehearsal in Asia

To be ready for the April occultation, the Langley team did a kind of general rehearsal in November 2024. Thanks to telescopes in Japan and Thailand they managed to calibrate their instruments, adjust predictions and fine-tune the exact location of Uranus with a precision of just 200 kilometres, which was key to nail the observation of April almost to the second.

Unveiling secrets

Although the main focus was to better understand the atmosphere of Uranus, the researchers also took advantage of this light curve to study the dynamics of its rings, measure turbulence in its atmosphere and improve the precision of its orbit. There was time for everything.

What are they aiming for with this?

To continue unveiling Uranus’s secrets, now, with these data, we can know if the climate has changed in the last decades or if it remains the same as 40 years ago. And for future missions? Well, it also helps to be able to program them or at least prepare them.

Let us remember that Uranus is almost 3.2 billion kilometres from Earth and that its atmosphere is mainly composed of hydrogen, helium and methane. Although it has 13 known rings, it is still a very mysterious planet. Its interior is a dense and icy soup of water, ammonia and methane in liquid state, that is why it is called an ice giant!

What is next?

Well, this does not end here. Over the next years, Uranus will occult several stars but it will be more tenuous. NASA is already looking towards 2031, when an even brighter occultation than the one in April this year will occur. Who knows what they will discover by then?