Distant Red Giant Caught Consuming One of Its Planets

An international team of astronomers believes it may have discovered a red giant star in the Perseus Cluster that is now consuming its innermost extrasolar planet. Their observations suggest that more lithium is present around the stellar object than accountable for through other explanations.

Stars in a certain mass range turn into red giants in their post-main sequence phase, as they are about to finish burning up their hydrogen supplies, and before switching to helium as a main fuel source.

The Sun will become a red giant as well, in about 4,5 billion years or so. When this happens, it will increase in size until its atmosphere engulfs the orbits of Mercury, Venus, Earth, and potentially Mars.

Observations of the red giant BD+48 740 were carried out via the Penn State – Torun Planet Search (PTPS), using the Hobby-Eberly Telescope (HET). The latter was equipped with the High Resolution Spectrograph (HRS) instrument at the time.

The object is believed to be older than our Sun, and has a radius about 11 times larger, say researchers from the Nicolaus Copernicus University (NCU) Torun Center for Astronomy, in Poland, the Universidad Autonoma de Madrid, in Spain, and the Pennsylvania State University, in the US.

Spectrographic analysis of the star found a lot more lithium in and around it than theoretically possible. Team experts believe that this is one of the strongest arguments that a small world is currently being destroyed, and its lithium reabsorbed into its parent star.

Another factor that strengthens this scenario is the orbit of a gas giant around the star. The exoplanet tips the scales at 1.6 Jovian masses, and has the highest-eccentricity orbit ever found in a post-main sequence stellar system.

“We discovered that this planet revolves around the star in an orbit that is only slightly wider than that of Mars at its narrowest point, but is much more extended at its farthest point,” NCU astronomer and study team member Andrzej Niedzielski says, quoted by Space.

“Such orbits are uncommon in planetary systems around evolved stars and, in fact, the BD+48 740 planet's orbit is the most elliptical one detected so far,” the expert goes on to explain.

If the innermost planet plunged into BD+48 740, the gravitational repercussions on the entire star systems would have significant enough to throw the second exoplanet in its current orbit, experts say.

“The highly elongated orbit of the massive planet we discovered around this lithium-polluted red-giant star is exactly the kind of evidence that would point to the star's recent destruction of its now-missing planet,” UAM study team member Eva Villaver adds.

The team admits that the possibility it's proposing cannot be verified directly with existing methods. However, their paper, appearing in the latest online issue of the esteemed Astrophysical Journal, does eliminate the vast majority of other potential explanations.