Casual Astronomer

Astronomers are getting a rare opportunity to watch what could be described as a comet being born. A distant icy object known as 450P/LONEOS has begun displaying increasingly obvious comet-like behaviour, potentially providing scientists with an important missing link between inactive centaurs and short-period comets.

Located billions of miles away in the outer Solar System, 450P/LONEOS was originally discovered in 2004 by the Lowell Observatory survey. It belongs to a fascinating population of objects known as centaurs, which generally travel through the region between Jupiter and Neptune.

These icy bodies are believed to originate farther out in the Solar System, potentially from the Kuiper Belt. Their interactions with the giant planets can gradually change their trajectories, sometimes pushing them towards the inner Solar System.

For 450P/LONEOS, that appears to be exactly what has happened.

From Centaur to Comet

Centaurs occupy an interesting position in the evolution of small Solar System bodies. Their orbits interact with those of the giant planets and can consequently remain unstable over astronomical timescales.

Eventually, encounters with Jupiter, Saturn, Uranus or Neptune can dramatically alter their futures. Some may be thrown out of the Solar System entirely, while others can be pushed closer to the Sun.

Scientists have long suspected that some centaurs eventually evolve into Jupiter-family comets. These are short-period comets whose orbits are strongly influenced by Jupiter.

450P/LONEOS has not quite reached that classification. Its current orbital period is approximately 22 years, whereas Jupiter-family comets typically complete an orbit in less than 20 years.

However, something remarkable is happening.

Astronomers using the Gemini North telescope in Hawaii have observed a cloud of gas and dust — known as a coma — developing around 450P’s nucleus. Observations between 2019 and 2024 showed this coma becoming brighter as the object approached perihelion, its closest point to the Sun.

Saturn May Have Changed 450P’s Destiny

Researchers tracing the object’s orbit backwards discovered a particularly important event.

In 1992, 450P passed within approximately 4.6 million kilometres of Saturn. On the scale of the Solar System, that was a remarkably close encounter.

Saturn’s gravity significantly altered 450P’s orbit and shortened the distance of its closest approach to the Sun. Its perihelion now occurs at roughly 5.4 astronomical units from our star, placing it only slightly beyond Jupiter’s average orbital distance.

When 450P reached perihelion again in August 2024, it consequently experienced considerably more solar heating than it would have before its encounter with Saturn.

That additional energy appears to be waking the icy body up.

JWST Discovers What Is Driving the Activity

The James Webb Space Telescope has provided scientists with an even closer look at what is happening around 450P.

Observations of its developing coma detected carbon dioxide gas along with indications of crystalline water-ice particles and dust. Interestingly, researchers did not detect water vapour.

That distinction is important.

At 450P’s current distance from the Sun, temperatures remain too low for ordinary water-ice sublimation to explain the activity. Instead, the evidence suggests that carbon dioxide is driving the formation of its coma.

As the nucleus warms, volatile material beneath or on its surface can escape into space. This escaping gas can carry dust and ice particles with it, gradually transforming what once appeared to be an inactive object into something increasingly resembling a comet.

Ancient Ice Is Being Changed

Scientists are also particularly interested in possible signs of crystalline water-ice.

Very primitive icy bodies can contain amorphous ice, in which water molecules do not possess the organised structure found in crystalline ice. This porous material can trap gases within it.

Heating can transform amorphous ice into a crystalline structure. During that process, trapped gases can escape and carry material away from the object’s surface.

Finding evidence of this transformation in 450P could therefore allow astronomers to watch material dating back towards the formation of the Solar System being physically altered as the object encounters stronger sunlight.

A Rare Glimpse Into Comet Evolution

Only a small proportion of known centaurs have displayed activity, making 450P/LONEOS particularly valuable to astronomers.

Rather than observing only the finished product in the form of an established comet, scientists may be witnessing one of the earlier stages of that transformation.

Another gravitational encounter with one of the giant planets could eventually alter 450P’s orbit further and potentially push it into the Jupiter-family comet population.

Whether that ultimately happens remains to be seen.

For now, 450P/LONEOS provides astronomers with something exceptionally valuable: an opportunity to investigate how an ancient, relatively inactive body from the outer Solar System begins responding to increased solar heating.

We normally think of comets as objects that simply exist and periodically sweep through the Solar System. With 450P, however, astronomers are getting the remarkable opportunity to watch how one of these spectacular objects may actually come to life.