Rick Robinson

Dec 11th 2018

A Ringside Seat for Planetary Formation


The theory of star and planetary formation tells us that newly formed planets should create gaps, produced as the growing planet sweeps up gas and dust. Astronomers have found visible gaps in the disks of gas and dust that surround young stars — tantalizing hints of planets taking form.

For the first time ever, astronomers have succeeded in taking a direct picture of a newly formed extrasolar planet, called PDS 70b.

The still-forming planet orbits PDS 70, a young star only about 10 million years old, located some 370 light-years from Earth. PDS 70b is already giant, weighing in at between five and nine times the mass of Jupiter, which takes about 120 years to orbit its parent star. A research team has detected hydrogen gas pouring onto the newborn world of PD70b, showing that planetary formation is still taking place, reported Sky & Telescope.

Picturing a Newborn World

Snapping a photograph of an infant extrasolar planet is a giant achievement in its own right — and one that reveals a remarkable wealth of information about this distant and very un-Earthlike world.

As Atlantic magazine noted, PDS 70b is scorching hot, about 1000 degrees K — much hotter than any planet in our own solar system. And according to the European Southern Observatory organization (ESO), examination of the world’s infrared spectrum indicates that the atmosphere of PDS 70b is cloudy.

Not content with these discoveries, a team of scientists immediately set to work making high-precision imagery using the 6.5-meter Magellan Clay Telescope located in Chile, as Sky & Telescope noted.

Unlike the original imaging of PDS 70b, which was taken in infrared light, Sky & Telescope explained that the Magellan Clay Telescope images were taken in red visible light, around the so-called hydrogen alpha band, with a wavelength of 656 nanometers. This is the wavelength of fluorescent light produced by heated hydrogen in space.

The purpose of the precision imaging was not to create a more detailed picture. At 370 light-years from Earth, even a giant planet like PDS 70b only appears as a bright dot in even the highest-resolution imaging we can produce. But the research team used the high-quality image to measure exactly just how bright and red the dot really is.

Lessons From a Bright Red Dot

The research team had to confirm, as they noted in their discovery report at, that the visible-light image at the very edge of detectability was the real thing, not a “false positive.” They reported that observations made during two nights in a row last May allowed them to reduce the chance of a false positive to less than one in a thousand.

The brightness and redness of hydrogen-alpha light is the telltale sign of what Sky & Telescope described as “shocked, hot, infalling hydrogen gas” — the gas swept up by PDS 70b in its orbit. In this description, “shocked” means that the gas has been suddenly heated and compressed by a shock wave as it falls toward the planet.

The researchers succinctly summarized their findings at “We conclude that PDS 70b is a young, actively accreting planet.”

According to AASNova, the measurements show that PDS 70b is accumulating hydrogen gas at the rate of one Jupiter mass in about 100 million years. Since the planet is only 10 million years old and already at least five times the mass of Jupiter, the flow rate must have been much heavier in the past.

This fits what the original image shows us: PDS 70b has already swept up most of the gas and dust along its orbit, producing the visible gap we see in the images. As a result, it has already grown, estimated Sky & Telescope, to about 90 percent of its full-grown mass. The infalling gas we detect now with hydrogen-alpha light is only the finishing touches, so to speak, of PDS 70b’s planetary formation.

But if this planet is almost completely formed, our study of extrasolar planet formation is still just beginning. What discoveries will we make with a new generation of space telescopes as well as new Earth-based telescopes?