The white dwarf stars are being polluted by debris from asteroid-like objects falling onto them, researchers said.
This discovery suggests that rocky planet assembly is common in clusters, say researchers. The stars, known as white dwarfs - small, dim remnants of stars once like the Sun - reside 150 light-years away in the Hyades star cluster, in the constellation of Taurus. The cluster is relatively young, at only 625 million years old.
Astronomers believe that all stars formed in clusters.
However, searches for planets in these clusters have not been fruitful - of the roughly 800 exoplanets known, only four are known to orbit stars in clusters.
A new study led by Jay Farihi of the University of Cambridge, UK, observed "retired" cluster stars to hunt for signs of planet formation.
Hubble's spectroscopic observations identified silicon in the atmospheres of two white dwarfs, a major ingredient of the rocky material that forms Earth and other terrestrial planets in the Solar System.
This silicon may have come from asteroids that were shredded by the white dwarfs' gravity when they veered too close to the stars.
The debris detected whirling around the white dwarfs suggests that terrestrial planets formed when these stars were born. After the stars collapsed to form white dwarfs, surviving gas giant planets may have gravitationally nudged members of any leftover asteroid belts into star-grazing orbits.
"We have identified chemical evidence for the building blocks of rocky planets. When these stars were born, they built planets, and there's a good chance that they currently retain some of them," said Farihi.
"The signs of rocky debris we are seeing are evidence of this - it is at least as rocky as the most primitive terrestrial bodies in our Solar System," Farihi said.
Besides finding silicon in the Hyades stars' atmospheres, Hubble also detected low levels of carbon. This is another sign of the rocky nature of the debris, as astronomers know that carbon levels should be very low in rocky, Earth-like material.
"Based on the silicon-to-carbon ratio in our study, for example, we can actually say that this material is basically Earth-like," Farihi said.