Sednoid

A sednoid is a specific type of trans-Neptunian object located in the far reaches of our solar system. These objects are defined by having very large orbits and highly eccentric paths. An eccentric orbit is one that is shaped like a stretched egg rather than a circle.

Astronomers also call them detached objects. This means they are so far from the Sun that the gravity of Neptune cannot strongly influence their movement. Because they stay so distant, some scientists believe they belong to the Inner Oort Cloud, also known as the Hills cloud. This region sits between 1,000 and 10,000 AU from the Sun.

To understand how a sednoid moves, we must look at its orbital mechanics. A sednoid has a large semi-major axis, which measures the average distance from the Sun. It also has a distant perihelion, which is the point in its orbit closest to the Sun. For these objects, the perihelion is always greater than 50 AU. This distance ensures they remain detached from the giant planets. Their high eccentricity is usually greater than 0.7. This extreme shape means they spend most of their time very far away from the Sun. They only swing closer during a small part of their incredibly long journeys.

Currently, astronomers agree on only four objects that belong to this population. The first is the namesake of the group, Sedna.

The other three known members are Biden, 541132 Leleākūhonua, and Ammonite. Each member has unique characteristics. For example, Leleākūhonua was announced in 2018 and has a semi-major axis of 1,090 AU. Its aphelion, or the farthest point from the Sun, is over 2,100 AU. This makes its orbit even larger than Sedna's.

While they share similar orbital traits, they may not all come from the same place. The spectral slope of Sedna is very different from that of Leleākūhonua, suggesting a heterogeneous origin.

The history of these discoveries has changed our view of the outer solar system. Sedna was discovered in 2003, providing the first real evidence of this population.

In 2015, Leleākūhonua was discovered, and it was later announced in 2018. These discoveries have led scientists to search for more members. In 2007 and 2008, a survey by Brown, Rabinowitz, and Schwamb searched for other sednoids. While they found the dwarf planet Gonggong, they did not find new sednoids. However, computer simulations suggest that about 40 Sedna-sized objects might exist in this distant region.

There are many significant numbers associated with these distant worlds. Sedna has an orbital period of about 11,400 years.

This means it takes over eleven millennia to complete one trip around the Sun. Leleākūhonua has a massive semi-major axis of 1,090 AU. Scientists estimate there may be about 2 million Inner Oort Cloud objects larger than 40 km. The total mass of these objects might be between 0.01 and 0.1 Earth masses. This is roughly the mass of Pluto and is several times larger than the asteroid belt.

The orbits of sednoids are a great mystery because they cannot be explained by known planets. The giant planets or galactic tides cannot account for their paths. Scientists have proposed several hypotheses to explain this. One idea is that a passing star lifted their orbits when the Sun was in a birth star cluster. They might have been captured from passing stars during that time. Another theory is that an unknown planet-sized body, often called Planet Nine, disrupts their orbits.

A temporary rogue planet in the early solar system could also have been responsible.

Studying sednoids connects us to the very beginning of our solar system. Astronomer Mike Brown called Sedna a "fossil record" of the early solar system. By studying these objects, we can learn how the Sun formed. We can also learn how many stars were close to our Sun in the past. If a hidden planet like Planet Nine exists, it would cause these objects to move in specific ways. Understanding the sednoids helps us map the invisible structures and history of our cosmic neighborhood.