Astronomers recently discovered a new rogue planet and, unlike previously discovered similar objects, were able to calculate both its mass and distance from Earth. A new study published in the journal Science describes how several successful observations from ground-based and space telescopes made these calculations possible.
Problem with detecting rogue planets
The methods used to search for other extrasolar planets depend largely on the parent stars. Of course, without a parent star, these methods cannot be used to search for a planet. In addition, planets do not emit their own light like stars, which makes them virtually invisible.
The only way astronomers can detect rogue planets is through microlensing, caused by the object’s slight gravitational effect on background light. This occurs when light from a distant star suddenly appears magnified to observers (telescopes on Earth), as if a lens had been placed in front of it. The magnification of the light allows astronomers to understand that something has passed in front of the distant star.
Theoretically, microlensing allows us to calculate the mass of an object passing in front of a star by analyzing the degree of curvature and, consequently, the magnification of light. However, without knowing the distance to the object, astronomers face what is known as “mass-distance degeneracy,” meaning they cannot be certain of the mass, since the same microlensing curve can result from different combinations of mass and distance. Therefore, without knowing one of these properties, they cannot be sure of the other, which leads only to approximate estimates.
Successful geometry
The microlensing effect of this particular rogue planet was observed by several telescopes on Earth, as well as by the Gaia space telescope. After its discovery, it was named by two different groups, resulting in two names: KMT-2024-BLG-0792 and OGLE-2024-BLG-0516.
And thanks to the timing of this event, Gaia was in the perfect position to take measurements to calculate the distance to the planet. Observations from two different points and a small difference in the time of the light signal allowed the team to calculate the microlensing parallax and determine the distance.
“Serendipitously, the KMT-2024-BLG-0792/OGLE-2024-BLG-0516 microlensing event was located nearly perpendicular to the direction of Gaia’s precession axis. This rare geometry caused the event to be observed by Gaia six times over a 16-hour period, beginning close to peak magnification,” the study authors write.
Based on their data, they determined that the planet’s mass is approximately 22% of Jupiter’s mass or slightly less than Saturn’s mass. They calculated that the planet is located at a distance of about 3,000 parsecs (or just under 10,000 light-years). Spectral analysis showed that the star it passed in front of was a red giant.
Einstein’s desert and the origin of rogue planets
Previously discovered rogue planets were generally considered to be smaller in mass than Jupiter, which, according to researchers, indicates that these objects formed in the protoplanetary disk and were then ejected from it. Larger bodies have also been discovered floating freely in space, but these are most likely brown dwarfs — a type of failed star that is too massive to be a planet but not massive enough to become a star.
Previous microlensing events have revealed a gap in their radial distribution, known as the “Einstein desert,” which is known to separate planets from brown dwarfs. The team of scientists argues that this gap is logical, as more massive planets are less likely to be ejected due to dynamic processes.
Although previous cases of free planets (FFPs) did not have directly measured masses, statistical estimates indicate that they are predominantly objects with masses less than Neptune that are either not gravitationally bound or are in very wide orbits.
Such objects can form as a result of strong gravitational interactions in their planetary systems. Scientists have concluded that violent dynamic processes shape the demographics of objects with planetary masses, both those that remain bound to their host stars and those that are ejected and become free-floating.
According to phys.org
