Humans are cosmic mayflies. Our lives come and go quickly, only offering us glimpses of the slow evolution of the Universe. Human history is measured in centuries, while the birth and death of stars and planets take place over millions and billions of years.
For that reason, we will never see the formation of another solar system unfold before our eyes. Instead, astronomers hope to observe planet formation in all its stages, each marking a phase in star and planet birth. That tells us not just the story of other star systems, but offers a glimpse into our own deep history, the one we can never see.
Think of that when you look at the beautiful image opening this piece. This image is so incredible it looks more like an artist’s impression than something from a telescope, but it shows the disk of gas and dust surrounding the newborn star HL Tauri. The star itself is just awakening; the fusion of hydrogen into helium that makes it shine has probably only been going on a million years or so, based on the amount and colors of light the star emits.
But the most breathtaking aspects of the image are the rings and gaps in the disk, never imaged before in this much detail. The largest gaps likely contain protoplanets, which form by collecting gas, dust, and small meteoroid fragments, gradually clearing their orbit of that debris. The combination of those gaps and the young age of HL Tauri suggest planets may form more quickly than astronomers think. A million years is long by human standards—our species has been around less than 200,000 years, after all—but it’s a tiny slice of Earth’s 4.5 billion-year history, and even tinier compared with the age of the Universe.
In visible light this star system is completely shrouded in dust, its details hidden. The image above was constructed with longer-wavelength light, using the Atacama Large Millimeter/submillimeter Array (ALMA). Millimeter and submillimeter light lies at the transition between infrared and microwaves, and it’s perfect for seeing into the clouds surrounding newborn star systems.
Additionally, ALMA consists of up to 66 telescopes acting together. (The information available doesn’t tell how many of these were used to observe HL Tauri.) By arranging them at their largest separation, about 16 kilometers, astronomers obtain higher resolution than any single telescope can achieve. It’s possible, in other words, that alien astronomers using a similar telescope array could have seen a disk like HL Tauri’s around our nascent Solar System, more than 4 billion years ago.
How many planets are forming? It’s too soon to say, since they are too small to be seen even in this high-resolution image. The disk surrounding HL Tauri is much bigger than Neptune’s orbit, so any planet in the gaps would at least begin at a larger orbit than the major planets in the Solar System. Additionally, other gaps could be “resonances”: orbits where the combined gravity of the star and protoplanets drive matter out, concentrating it in the rings. (The Asteroid Belt in the Solar System has many such gaps, created by the gravity of the Sun and Jupiter.)
The census of other star systems suggest planets migrate after they form, thanks to friction with remaining gas and the gravitational influence of other planets in the system. The HL Tauri system we see today may not resemble the final form it takes after a few more million years. Over the last 15 years, we’ve learned that planetary systems are a mashup of physical laws and historical circumstances. The types of planets that form, where they orbit relative to the star and each other, and other aspects fall into certain patterns, but we see a lot of variety rather than identical systems.
HL Tauri is about 450 light-years away in the constellation of Taurus. We see the protoplanetary disk around it at an angle, but nearly “face-on.” Which is lucky: we can see the gaps in the disk more clearly than if the disk were at a steeper angle. It’s astounding to think that from 450 light-years’ distance, we can see distinct signs of newborn planets…and it’s unlikely HL Tauri will be the last such system we can observe.
We can’t see the birth of our own Solar System, though we can infer a lot about it from the debris it left behind: comets, meteoroids, and asteroids scattered around the major planets. But there’s an earlier time, when a giant cloud of gas and dust slowly collapsed under its own gravity. HL Tauri provides a glimpse into that era for another star, a single moment of history in an alien system that might possibly someday have its own civilization of astronomers who wonder how their planet formed so long before.