First, one needs eyes sensitive enough to detect the "needles." While stars are relatively bright and easy to spot, planetary-mass members are several thousand times fainter and can only be detected with large aperture telescopes and sensitive detectors. Identifying FFPs within a star cluster is a major challenge, in many ways, similar to the "needle in the haystack" parable. The nature and origin of FFPs has remained controversial since their discovery: do they form like stars through the gravitational collapse of small clouds of gas? Or are they formed around stars like other planets and are then dynamically ejected or stripped off? While it is known that both mechanisms can produce FFPs, their respective contributions are still an open question due to the lack of a large homogeneous sample.
Astronomers call these free-floating planets (FFPs). On the other hand, it has been known since around the year 2000 that there are isolated planets, not circling around the stars. These exoplanets are usually circling around their host stars. Now more than 4,500 planets have been discovered beyond our Solar System (exoplanets). Although small and light, they have attracted the attention of many people since long ago. Like Earth and Jupiter in our Solar System, planets are very low-mass objects compared with the Sun. They are not circling around a host star but are isolated like shown in this figure. Approximately 100 free-floating planets have been detected in the Upper Scorpius region in this study. Figure 1: Artist’s impression of a Jupiter-mass free-floating planet in a star forming region.