Trans-Neptunian objects (TNOs) are objects that orbit the Sun beyond Neptune. They mainly are found either as Kuiper belt objects (KBOs) or Oort cloud objects (OCOs). KBOs orbit the Sun within a region extending from the orbit of Neptune out to ~50 AU. Pluto is an example of a KBO and one of its largest members. Detecting KBOs, especially smaller bodies, is very challenging due to the huge distance and faintness of these objects. As a result, small KBOs with sizes on the order of a few km or less still elude detection even though they are expected to be far more numerous than larger KBOs. Beyond the KBOs are the OCOs. These objects comprise the Oort cloud, an enormous spherical cloud of icy bodies extending as far as 50,000 AU from the Sun, or nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun.
Figure 1: Artist’s concept of the Solar System in perspective. The scale bar is in astronomical units, with each set distance beyond 1 AU representing 10 times the previous distance. Credit: NASA/JPL-Caltech.
The Oort cloud is thought to be made of two separate regions: a spherical outer Oort cloud and a disc-shaped inner Oort cloud. No direct observations of the Oort cloud have yet been made. Nevertheless, a handful of presently known TNOs are possible members of the inner Oort cloud. One such object is Sedna - a large inner Oort cloud object about 1000 km in diameter. Sedna orbits around the Sun in an extremely elliptical orbit, coming as close as 76 AU from the Sun and receding out as far as 937 AU. Each orbit takes Sedna ~11,400 years. When Sedna was discovered in 2003, it was close to its minimum distance from the Sun. Given that an object like Sedna spends the vast majority of its time much further from the Sun; its discovery indicates there may be a large population of Sedna-sized objects out there.
Doressoundiram et al. (2013) propose a technique known as the serendipitous stellar occultation method to detect objects residing in the Kuiper belt and Oort cloud. It involves detecting such objects when they happen to pass in front of a star and occult it. This is well suited for detecting tiny and invisible objects that are otherwise too faint for current telescopes to photograph. In fact, the smallest KBO ever found is less than 1 km in size and it was found by the Hubble Space Telescope using just such a technique. Although the object is too faint to be photographed by the Hubble Space Telescope, it was detected when it passed in front of a background star, temporarily disrupting the starlight.
Figure 2: Artist’s impression of a small KBO in the process of occulting a background star. Credit: NASA, ESA and G. Bacon (STScI).
The instrument envisioned for the detection of TNOs by serendipitous stellar occultations is known as a Fast Multi-Object Photometer (F-MOP). It consists of several 3×3 bundles of optical fibres feeding a CCD. Each 3×3 fibre bundle would monitor a single star and the whole instrument would monitor a relatively wide field of view containing 50 to 100 stars that are suitable for detecting TNOs by stellar occultations. The F-MOP instrument is mounted onto an 8 m diameter telescope. The stars would be monitored with a cadence of at least 50 Hz to catch the occultation events. This is because, for a small TNO, an occultation event is expected to last for only about one second.
Figure 3: Synthetic light curve profile of an occultation event involving a 4 km object at 1300 AU. (Doressoundiram et al., 2013)
Consider a target star with an angular size of 0.015 milli-arcseconds. For such a star, the F-MOP instrument can detect an occultation event caused by an object 300 m or larger at 50 AU, or an object 840 m or larger at 200 AU. If the angular size of the target star is smaller, then the minimum size of an object that can be detected by occultation is also smaller. For an object at 5000 AU, it can be detected if it is 30 km or larger and occults a target star with an angular size of 0.014 milli-arcseconds. If the angular size of the target star is reduced by a factor of ten to 0.0014 milli-arcseconds, an object 4 km or larger can be detected if it occults. Hence, serendipitous stellar occultation can serve as a powerful and unique tool to probe the Kuiper belt and Oort cloud.
Doressoundiram et al., “Ground-based exploration of the outer Solar system by serendipitous stellar occultations”, MNRAS 428, 2661-2667 (2013)