Where do comets originate?

A new technique developed by team of Penn astronomers may allow scientists to measure radiation from celestial bodies that are only theorized to exist. Indeed, the team presents an intriguing detection of radiation that may well originate from the source of comets around nearby stars.

Nearly three-quarters of a century ago, Dutch scientist Jan Oort predicted that a cloud of icy bodies floated at the far reaches of the solar system and dispatched recurring comets like Haley’s. The existence of such an Oort cloud seemed likely, but no one could figure out how to prove it was real.

Oort clouds, both the one within our solar system that orbits the sun, and others that may exist that orbit other stars, reside at a great distance from a central star and are made of widely dispersed particles, making them difficult to detect. The Oort cloud in our solar system is believed to extend nearly a light year from the sun, with particles ranging in size from a few kilometers to mere centimeters or smaller, dispersed over a vast area. Looking out through the solar system to study these clouds poses a challenge.

Data from the CMB-mapping Planck satellite.
Map of Planck 857 GHz signal centered on Fomalhaut. Error bars shown take into account variations due to large-scale fluctuations in the galactic backgrounds.

“These little icy bodies essentially get kicked into the outer reaches of the solar system, interact with Jupiter, and then stabilize themselves on these long orbits at very, very far distances,” says Cullen Blake, an assistant professor of physics and astronomy, who collaborated with Penn astronomers Bhuvnesh Jain and postdoc Eric Baxter on a paper on the subject published in The Astronomical Journal.

To come at the challenge from a different angle, the researchers speculated that they could measure Oort clouds not around our sun but around other stars, using data that had been collected for a different purpose. They combed wide-area survey maps from the Planck satellite mission, searching for a signal of the ambient energy remaining after the Big Bang (known as the Cosmic Microwave Background, or CMB.)

In general, space is very cold, notes Blake, with “temperature set by leftover radiation from the Big Bang of 2 or 3 degrees Kelvin” or around -450 degrees Fahrenheit. These clouds, however, are slightly warmer, likely due to proximity to their sun.

Blake explains that a piece of ice floating through space that’s not near such a star would not experience this slight temperature increase. Rather, it would remain as cold as what’s around it. “Icy objects at those [warmer] temperatures emit infrared radiation that we can hope to see in aggregate,” he says. “So, we are looking to detect the total amount of infrared or microwave radiation from all these little bodies in the Oort clouds orbiting nearby stars.”

Just as a passenger in an airplane can distinguish the parameters of a faraway cloud better than one nearby, the Penn scientists determined their best chance was to search beyond our solar system. They investigated radiation from bright stars in the Milky Way galaxy. The team successfully detected such radiation from the debris disk of Fomalhaut, one of the brightest stars and part of the Piscis Austrinus constellation, though the researchers don’t know for certain how much, if any, came from an Oort cloud.

Next, the researchers say they hope to take advantage of data gathered from one of the long-duration balloon missions that NASA launches from Antarctica to evaluate the energy surrounding other stars. They would also like to figure out how to determine whether the bodies emitting that energy are gravitationally bound to the star they surround.

Blake specializes in the discovery of new planets orbiting stars, and his collaborators are experts in understanding the evolution of the universe. Putting these skills together to validate Oort’s theory, Blake says, has so far been the best part of the project.

The work was supported by funding from NASA and Penn’s Center for Particle Cosmology.

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