Astronomers watching a small red dwarf star 500 light years away were surprised to notice a brief dip in its already dim light. But they quickly identified the source of the change: the darker mass of a planet passing between the distant star and our vantage point on Earth.


The star HATS-6 is about half the size of our Sun, in both mass and diameter. It is one of the smallest stars astronomers have seen with a gas giant, and apparently, our models can’t yet explain how this gas giant came to be so close to its parent star.



The planet in question (HATS-6b) weighs in at approximately the mass of Saturn, or 100 times the mass of Earth. But because of its close orbital distance, the star’s heat has caused HATS-6b’s gas to billow out, inflating the planet like a hot air balloon to the size of Jupiter. With an orbital period of just 3.3 days, astronomers say HATS-6b is significantly closer to its star than the much smaller Mercury is to our Sun.


When solar systems develop, gas giants usually form in the outer regions, where gas and ice are more abundant, but they don’t always stay there. Sometimes, thanks to gravitational nudges from other objects, they migrate inward to orbit very close to their stars. This happens as a normal part of the process of planets settling down into stable orbits.



For instance, in our solar system’s early history, Neptune’s orbit was once closer to the Sun than Uranus, until an orbital resonance between Jupiter and Saturn created enough of a gravitational disturbance to shove Neptune outward. As Neptune stumbled into the smaller icy objects of the Kuiper Belt, its gravity pushed several of them inward. When these displaced Kuiper Belt objects encountered Jupiter, the gas giant’s gravity flung most of them out of the Solar System, but that interaction also nudged Jupiter inward slightly.



In other solar systems, astronomers have found planets similar to Jupiter orbiting much closer to their suns. Many of these exoplanets have closer orbits than Mercury’s path around our Sun, and the heat has caused their gases to expand. Scientists call these planets “Hot Jupiters,” and they have worked out models explaining how they formed further out in their solar systems and migrated inward toward their stars. HATS-6b appears to more like a “Warm Saturn,” as scientists called it in a recent paper published in The Astronomical Journal.



Most of the known Hot Jupiters orbit stars more like our Sun, however. They are larger and hotter than red dwarfs like HATS-6, which means they exert more gravitational force. Astronomers are not yet sure how a planet like HATS-6b could have migrated into such a close orbit around a small star like HATS-6.




Red dwarfs, also called M-dwarf stars, are small, slow-burning stars with long lives even by astronomical standards. They are cool and dim, which makes them harder to see with telescopes than brighter stars; HATS-6, for example, is only about 5 percent as bright as our Sun. However, scientists think they may account for almost 75 percent of the stars in the Milky Way Galaxy. [The Astronomical Journal]