Watching a Planet’s Birth in Real Time

Your friendly neighborhood geologist will tell you that the age of the Earth is 4.54 billion years, give or take 45 million.

Since modern humans have been around for only about 60,000 years of that time, it’s hard for us to even guess at how exactly the planet was born.

Luckily we have a variety of tools at our disposal to make sure we’re making highly educated guesses, including orbiting observatories like the Spitzer Space Telescope.

Using its infrared vision to peer through dust and thick gases, Spitzer has seen plenty of evidence for young star systems taking shape since it was launched in 2003. But most of that evidence has been fairly static, considering that it takes planets millions of years to develop.

Now, in a rare catch, astronomers using Spitzer think they’ve witnessed an early stage of planet formation in real time.


—Image courtesy NASA/JPL-Caltech/R. Hurt (SSC)

The team watched one young star, LRLL 31, for five months, recording changes in its infrared light. The star had previously been called out for having a type of debris ring known as a transitional disk.

According to a popular theory of planet formation, some stars are surrounded by thick disks of dust and gases. Over time, larger grains within these disks start to collect material, and, like rolling snowballs, they grow larger as they pull more material unto themselves.

At some point, objects get so large that they carve gaps in the original disk, creating what’s known as a transitional disk.

Spitzer showed that LRLL 31 has such a disk with both an inner and outer gap. What’s more, the infrared light from the inner disk changes its brightness and wavelength every few weeks.

The team thinks the changes are due to a “companion”—some body circling the star inside the inner gap. As it orbits the star, this body pushes the disk’s material around like a cornering boat pushes water, creating a “wave” that periodically changes the disk’s height.

Higher waves facing Earth mean more and hotter material reflecting the host’s starlight, so more infrared radiation and at shorter wavelengths. The wave also casts its shadow on the outer disk, blocking its longer-wavelength light.

The opposite scenario is true when the wave crests between Earth and the star.

Astronomers aren’t 100 percent sure if the body creating the wave is really a developing planet or some other companion, maybe even another star.

But lead study author James Muzerolle, of the Space Telescope Science Institute in Baltimore, notes in a statement: “For astronomers, watching anything in real-time is exciting. It’s like we’re biologists getting to watch cells grow in a petri dish, only our specimen is light-years away.”

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