In 1995, humanity crossed an invisible threshold in its quest to understand the cosmos. For the first time, astronomers confirmed the existence of a planet orbiting a star beyond our solar system. That discovery shattered centuries of speculation, opening a universe that suddenly seemed far richer and stranger than we had ever imagined. Today, less than three decades later, we know of more than 6,000 exoplanets—alien worlds orbiting distant suns. Some are giants larger than Jupiter, others are scorched by the searing light of their stars, and still others are frozen far beyond the warmth of their parent systems. Their diversity is staggering, far exceeding the boundaries of the eight familiar planets in our own solar system.
This great cosmic census forces us to confront questions that reach to the heart of our existence. How do such planets form and evolve? What strange processes shape their sizes, orbits, and atmospheres? And among this galactic menagerie, which of them might resemble Earth closely enough to host life? The answers to these questions lie not only in studying fully formed planets but in catching them at the very moment of birth, within the dusty wombs of their stars.
The Cradle of Planets
Every planet—whether rocky like Earth or gaseous like Jupiter—begins life in a protoplanetary disk. These disks are delicate, rotating structures of gas and dust that swirl around newborn stars. To the eye of a powerful telescope, they often appear as glowing halos or faint rings, silent witnesses to the drama of creation unfolding within them.
In these disks, dust grains collide and stick together, gradually forming pebbles, rocks, and eventually the building blocks of planets. Small rocky planets emerge near the warmth of their star, while gas giants grow farther away, pulling in vast envelopes of hydrogen and helium. Over millions of years, entire planetary systems are sculpted from these fragile, ephemeral disks.
Protoplanetary disks are not rare. They have been observed around young stars of many kinds, from small red dwarfs to stars much more massive than our Sun. Yet the process of watching a planet actually take shape inside its natal disk is extraordinarily difficult. Young planets are often hidden within the thick dust, revealing themselves only when their gravity carves gaps or spirals in the disk around them.
A Revolution in Vision
Since the 2010s, a new generation of telescopes has transformed our understanding of planet formation. The Subaru Telescope in Hawaii, with its ability to capture visible and infrared light, and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, which peers into radio wavelengths, have peeled back the layers of protoplanetary disks with unprecedented clarity. These instruments revealed breathtaking details—rings, gaps, and spiral arms that hint at the invisible presence of newborn worlds.
But inferring the existence of planets from the patterns they leave behind is not the same as directly seeing them. Direct detections of forming planets—protoplanets—have been exceedingly rare. Until recently, only a handful of examples were known, such as PDS 70 b and c, and the intriguing candidate AB Aurigae b (AB Aur b).
The rarity of such detections underscores the difficulty. Most protoplanets remain cloaked within the dense material of their disks, invisible to conventional observation. Only when they grow massive enough to disturb their surroundings, or when conditions allow astronomers to look straight into the heart of a disk, do these hidden worlds reveal themselves.
The Growing Pains of Worlds
A crucial step in understanding planetary birth is to observe how these young worlds gather their mass. Protoplanets are not static; they are hungry, constantly pulling in material from the surrounding disk. This accretion process fuels their growth and determines their ultimate fate. Yet capturing the details of this process has been nearly impossible.
The PDS 70 system provided the first tantalizing glimpses, where astronomers observed hydrogen emission signaling material falling onto the planets. But most systems remained elusive—until AB Aur b.
The Breakthrough of AB Aur b
In a remarkable achievement, an international team of researchers led by the Astrobiology Center in Japan and the University of Texas at San Antonio succeeded in capturing evidence of ongoing accretion in AB Aur b. Using the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT) in Chile, they detected hydrogen emission lines at the location of the protoplanet.
These emissions are not random. They carry a signature known as an inverse P Cygni profile, a spectral fingerprint that reveals gas streaming inward. This is the unmistakable sign of a planet in the act of feeding, material spiraling down from a small disk surrounding the young world onto the planet itself.
AB Aur b is unlike anything in our solar system. It is about four times the mass of Jupiter, a true giant, orbiting its star at an astonishing distance of 93 astronomical units—more than twice as far as Pluto is from the Sun. No planet in our solar system lies so distant, nor is any so massive in such an orbit. Its existence challenges conventional models of planet formation, which predict that such large worlds cannot easily form so far from their parent star through the slow accumulation of dust and gas.
Instead, AB Aur b may have been born through a more dramatic process: gravitational instability. In this scenario, the protoplanetary disk itself becomes so massive and unstable that it fragments under its own gravity, collapsing directly into giant planets. If so, AB Aur b represents a fundamentally different pathway of planet formation, one that creates worlds far larger and farther from their stars than the ones we know.
Expanding the Horizon of Possibility
The discovery of AB Aur b in its formative state is more than an isolated triumph. It is a window into the staggering diversity of planetary systems. Our solar system, with its neat architecture of rocky inner planets and distant gas giants, is only one arrangement among countless possibilities. Elsewhere, giant planets can form at vast distances, scorching “hot Jupiters” can orbit closer than Mercury, and small rocky worlds can circle red dwarf stars where conditions might still allow liquid water to exist.
Such diversity broadens our imagination of what worlds might look like—and where life might take hold. Not every planet needs to be a twin of Earth to foster habitability. Some may have thick atmospheres that trap warmth, underground oceans shielded by ice, or exotic chemistries that give rise to unfamiliar forms of life. Each new discovery expands the boundaries of what “Earth-like” could mean.
Looking Toward the Future
The quest to observe planets in the act of forming is only just beginning. With the launch of the James Webb Space Telescope (JWST), astronomers now have an even sharper tool to peer into dusty disks and detect the faint heat signatures of hidden worlds. Future instruments on Earth, such as the Extremely Large Telescope (ELT), promise to resolve details at scales unimaginable just a decade ago.
These tools will allow us to study not only the massive, exotic giants like AB Aur b but also smaller, rocky planets more akin to Earth. They will help us trace the processes that sculpt planetary atmospheres, dictate orbital paths, and set the stage for life itself.
The Human Meaning of Planetary Birth
Behind the technical details and spectral profiles lies a profound truth: when we observe a planet forming, we are watching the universe give birth to new worlds. The gas, dust, and hydrogen emissions are more than data—they are the heartbeat of creation, echoing the very process that once gave rise to Earth.
To witness a protoplanet like AB Aur b is to glimpse the story of our own origins, written in the language of physics and light. It reminds us that our planet, our oceans, our skies, were once just dust around a young star. The discovery of other planets in the making connects us to a cosmic lineage that stretches across time and space.
And within that lineage lies a question that may be the most profound of all: if worlds are still forming everywhere in the galaxy, how many of them, like Earth, will one day cradle life?
More information: Thayne Currie et al, VLT/MUSE Detection of the AB Aurigae b Protoplanet with H α Spectroscopy, The Astrophysical Journal Letters (2025). DOI: 10.3847/2041-8213/adf7a0
