This artist’s impression shows the orbits of planets and comets around the star 61 Vir, superimposed on a view from the Herschel Space Telescope. Credit: ESA/AOES

A European space telescope has stared deep into two star systems known to host exoplanets and made a rather cool discovery — both systems are surrounded by comets.

The Herschel Space Observatory views the cosmos in infrared light, and during a recent campaign it studied the infrared glow surrounding the star systems Gliese 581 and 61 Virginis that are located 22 and 28 light-years from Earth, respectively. The M-class dwarf Gliese 581 recently shot to fame as a star hosting a system of at least four worlds, one of which is known to orbit inside the star’s habitable zone. The G-class 61 Virginis is known to play host to two “super-Earths”; one is twice the mass of our planet and the other is 18-times the mass.


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It’s exciting to find multiplanet star systems as this suggests that our solar system is not a rare occurrence. But now, with the help of Herschel, another solar system trait appears to be common.

The solar system is surrounded by a ring of frozen debris extending beyond the orbit of Pluto. Known as the Kuiper Belt, this debris field is known to be the origin of some of the comets that plunge through the inner solar system. The gravitational interaction with Jupiter and Saturn caused cometary embryos — huge chunks of ices and rock — to become dislodged from this belt early in the solar system’s history, sparking a barrage of comet strikes of the inner planets, including Earth. This process is thought to have delivered water to our planet.

But the interesting thing about Gliese 581 and 61 Virginis is that neither appear to contain large gas giant worlds — the largest is a Neptune-class world. This means that the Kuiper Belt-like features surrounding both stars are heavily populated with cometary embryos that never left home.

Artist impression of the debris disc and planets around the star known as Gliese 581, superimposed on Herschel PACS images at 70, 100 and 160 micrometer wavelengths. Credit: ESA/AOES

After careful analysis, the space telescope deduced that the cometary belts are ten-times more populated than our Kuiper Belt. It did this by observing the faint infrared glow of dust contained inside the star systems’ belts.

“The new observations are giving us a clue: they’re saying that in the solar system we have giant planets and a relatively sparse Kuiper Belt, but systems with only low-mass planets often have much denser Kuiper belts,” says Mark Wyatt from the University of Cambridge, UK, lead author of the 61 Virginis paper.

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Herschel is no stranger to detecting evidence of comets in other star systems. Earlier this year the telescope trained its eye on the famous star Fomalhaut.

By looking at the infrared radiation from its vast dusty disk, astronomers were able to estimate how many comets must be continually destroyed (through comet-comet collisions and strikes with hypothetical planetary bodies) to supply the dust surrounding the star. 260 billion to 83 trillion comets are thought to be bumping around in Fomalhaut’s belt, a similar number of comets that are thought to be contained in the solar system’s Oort Cloud.

These new observations hold a special relevance to the origins of planetary oceans, and by extension, the habitability of Earth and the potential habitability of exoplanets. The absence of large Saturn- to Jupiter-mass worlds means that these star systems likely avoided the heavy bombardment of comets the inner solar system received and, instead, are experiencing a steady rain of comets. The comets of Gliese 581 and 61 Virginis are in it for the long-haul, laying siege.

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“We think that may be because the absence of a Jupiter in the low-mass planet systems allows them to avoid a dramatic heavy bombardment event, and instead experience a gradual rain of comets over billions of years,” added Wyatt.

“For an older star like GJ 581, which is at least two billion years old, enough time has elapsed for such a gradual rain of comets to deliver a sizable amount of water to the innermost planets, which is of particular importance for the planet residing in the star’s habitable zone,” said Jean-Francois Lestrade of the Observatoire de Paris who led the Gliese 581 research.