Peering Into a Planet Nursery Using Radio Waves

Both the Atacama Large Millimeter Array (ALMA) and the Karl G. Jansky Very Large Array (VLA) looked at the baby star HL Tau to search out the beginnings of planet formation. ALMA image of HL Tau at left; VLA image, showing clump of dust, at right.
Carrasco-Gonzalez, et al.; Bill Saxton, NRAO/AUI/NSF.

Imaging that you could use giant radio telescopes to peek into the birthplaces of planets. It's not a futuristic science-fiction dream: it's a regular occurrence as astronomers use radio observatories to take a sneak peek at star and planet birth. In particular, the Karl G. Jansky Very Large Array (VLA) in New Mexico has looked at a very young star called HL Tau and found the beginnings of planet formation.  

How Planets Form

When stars like HL Tau (which is only about a million years old—a mere infant in stellar terms) are born, they are surrounded by a cloud of gas and dust that was once the stellar nursery. The dust particles are the building blocks of planets, and start to coalesce within the larger cloud. The cloud itself flattens out into a disk shape surrounding the star. Eventually, over hundreds of thousands of years, large clumps form, and those are the infant planets. Unfortunately for astronomers, all that planet-birthing activity is buried in the dust clouds. That makes the activity invisible to us until the dust clears out. Once the dust does dissipate (or is gathered up as part of the planet-forming process), then the planets are detectable. This is the process that built our solar system, and is expected to be observed around other newborn stars in the Milky Way and other galaxies.

So, how can astronomers observe details of planet birth when they're hidden inside a thick cloud of dust. The solution lies in radio astronomy. It turns out that radio astronomy observatories such as the VLA and the Atacama Large Millimeter Array (ALMA) can help.

How Do Radio Waves Reveal Baby Planets?

Radio waves have a unique property: they can slip through a cloud of gas and dust and reveal what lies inside. Since they penetrate dust, we use radio astronomy techniques to study regions that cannot be seen in visible light, such as the dust-shrouded, busy center of our galaxy, the Milky Way. Radio waves also allow us to trace the location, density, and motion of the hydrogen gas that constitutes three-fourths of the ordinary matter in the universe. In addition, such waves have been used to penetrate other clouds of gas and dust where stars (and presumably planets) are born. These starbirth nurseries (such as the Orion Nebula) lie throughout our galaxy, and give us a good idea of the amount of star formation going on throughout the Milky Way.

More about HL Tau

The infant star HL Tau lies about 450 light-years from Earth in the direction of the constellation Taurus. Astronomers have long thought that it and its forming planets were long thought to be a good example of the activity that formed our own solar system 4.6 billion years ago. Astronomers looked at the star and its disk in 2014, using ALMA. That study provided the best radio image of planet formation in progress. Moreover, the ALMA data revealed showed gaps in the disk. Those are probably caused by planet-like bodies sweeping out the dust along their orbits. 

The ALMA image showed details of the system in the outer portions of the disk. However, the inner parts of the disk were still enveloped in dust that was hard for ALMA to "see" through. So, astronomers turned to the VLA, which detects longer wavelengths. 

The new VLA images did the trick. They revealed a distinct clump of dust in the inner region of the disk. The clump contains somewhere between three and eight times the mass of planet Earth, and is at the earliest stage of planet formation ever seen. The VLA data also gave astronomers some clues about the makeup of the dust particles in the inner disk. Radio data shows that the inner region of the disk contains grains as large as a centimeter in diameter. These are the smallest building blocks of planets. The inner region is presumably where Earth-like planets will form in the future, as clumps of dust grow by pulling in material from their surroundings, growing larger and larger over time. Eventually, they become planets. The leftovers of planet formation become asteroids, comets, and meteoroids that will likely bombard the newborn planets during the early history of the system. That's what happened in our own solar system. Thus, looking at HL Tau is very much like looking at a birth snapshot of the solar system.