Host: Aqib Khan; Text: Xie Jinlong; Photo: Ahmed Mohamed Reda; Video: Xu Lei; Reviewer: Deng Qingyun,Li Xuerou, Stephen C. McClure
About the speaker
Arika Higuchi, a researcher worked in Celestial mechanics and planetary sciences, is from National Astronomical Observatory of Japan (NAOJ). She got her doctorate degree in science from KOBE University. Her research and development topics include the orbital dynamics of celestial objects, especially small Solar system bodies.
About this English GeoScience Café session
At 19:00, on 19 December 2018, Arika Higuchi attended English GeoScience Café No.28 and spoke about Solar System Bodies and the Oort cloud. At the beginning, the host gave a quick prologue of the session and introduced the speaker. Arika Higuchi expressed some appreciation about EGSC and started her talk. She then outlined her presentation and divided it in two parts as follows:
Part1: Solar System
Part2: Formation of the Oort cloud
Fig.1. Planet, comet, and asteroid
Part 1 Solar System
1.1 Definition of Solar System bodies
First off, Arika Higuchi introduced the definition of Solar System bodies. She noted that Solar System bodies include planets, dwarf planets, satellites and small Solar system bodies. There are eight planets in the Solar System and a planet is a massive round body that orbits the sun. A dwarf planet is similar to a planet, but smaller. Planets capture their neighborhood, drawing smaller bodies around themselves while dwarf planets do not have this capacity. A satellite is a small body orbiting a planet or a smaller body. Other bodies in the Solar system including asteroids, comets, and the Oort cloud are referred to as small Solar system bodies.
Arika Higuchi also mentioned some units. As she said, scientists often use astronomical unit, “au”, to describe the distance. Sun-Earth distance is 1 au, Sun-Jupiter distance is 5.2 au while Sun-Neptune is 30 au. There are also several units of weight, solar mass (2×10^30 kg), Jupiter mass and Earth mass. A Jupiter mass is 1/1000 of solar mass. An Earth mass is 1/300 of Jupiter mass.
1.2 Solar system Planet formation
Arika went on to talk briefly about the formation of planets in our solar system. At the beginning, solar system had only a proto-Sun and proto-planetary disk, formed from a molecular cloud consisting of gas and solid dust grains. Gradually, dust grains grew in mass and formed into planetesimals. Over time, the mass of these planetesimals grew into planets. Planets accreting gas before the disk-gas disappeared became gaseous planets. Planetesimals leftover after planet formation became small solar system bodies.
Fig.2. Evolution of the solar system
1.3 Small solar system bodies
Arika also introduced the small solar system bodies. The main asteroid belt is between Mars and Jupiter, while objects beyond Neptune are called Trans-Neptunian Objects (TNOs). The objects around Earth are called Near Earth Objects (NEOs). Between Jupiter and Neptune there are also some small bodies called Centaurs. Although most of the small solar system bodies are between Earth and Jupiter, there are also some small bodies, comets, that are far from the Sun, more than 1000 au which are called the Oort cloud, a reservoir of long period comets.
It is interesting that some small bodies were born where they exist right now while some were born somewhere else and were transported the place where they are today. The main belt asteroids and Trans-Neptunian objects were native and just born where they are. Near Earth objects were born somewhere else and transported to where they are today, around the earth. It was just because of the unstable orbit in the region around the earth affected by the gravity of earth and other planets, like Mars and small bodies, which forced their transport to other places. There were some leftovers around the earth 4.5 billion years ago, but they were ejected somewhere else so the objects in this region are relatively new, as they were recently transported from somewhere else. We think that the main asteroid belt is the source of Near Earth objects. The dynamic life of these new small bodies is less than 10^7
years, a short time relative to 4.5 billion years. Another unstable region is the region between Jupiter and Neptune with four giant planets nearby with dynamic lifetime of less than 10^7 years. Many objects in this region were transported from somewhere else quite recently, and like the large Oord cloud, were transported from somewhere else. The Oord cloud is far from the Sun, so the ingredients necessary to form a planetesimals are insufficient, so planetesimals must have formed in another part of the solar system and then were transported to their current position.
The Dutch astronomer, Jan Oort, in 1950 first proposed the idea of a cloud surrounding the solar system, what we now call the Oort cold. It is a reservoir of long-period comets surrounding the Solar system; these comets are really planetesimals that formed in the planetary region.
Fig.3. The Oort cloud is surrounding the Solar system
After planet formation, there were many planetesimals around the Jupiter. These planetesimals were transported by the gravity from Jupiter into the distant regions, becoming the Oort cloud. Although the main planetesimals were ejected into the interstellar space, what we call free-floating comets without orbiting any stars; these objects float in interstellar space. Some of the free-floating comets were objects ejected into the interstellar space by the gravitation of stars, then becoming part of the Oort cloud. There are also many planetesimals ejected from other systems in the universe. We may observe them if we are lucky.
Part 2 Formation of the Oort cloud
Arika introduced some import orbital elements. Orbital elements are parameters that describe orbits. The most important orbit parameter is semi-major axis, defined as one-half of the long axis. The shorter distance between the Sun and the point of least distance is called perihelion distance, and the longest distance is called the aphelion point. The distance between the mass center and the Sun is called aphelion distance. Eccentricity is another parameter to describe the shape of an orbit. Following the equation (Fig.4), if we know two of the parameters, we can derive all of them.
Fig.4. Relationship between four ellipse elements
Inclination is an angle between the orbital plane and the reference plane (ecliptic plane), which also is an orbital element. Eccentricity affects the orbit shape. When the eccentricity is near one (Fig.5), the aphelion distance is almost twice the length of the semi-major axis.
Fig.5. Orbit shapes with different eccentricities
2.2 Comet and Asteroid Orbital Element Distribution
Arika talked about the distribution of comet and asteroid orbital element. As Fig.6 shows, every dot in orange or blue means an asteroid or a comet. Most of the dots are between Mars and Jupiter, and follow a non-uniform distribution. Many of them have a small eccentricity. The rectangle marks the region of the main asteroid belt.
Fig.6. Inner solar system comet and Asteroid orbital element distribution
The semi-major axis of the main asteroid belt is between 2 to 3.5 au. Most of their eccentricities are smaller than 0.3. In Fig.7, there are some gaps, shown in white. The orbits in these regions are very unstable with no asteroids. This instability stems from the influence of the gravity of Jupiter. We call these regions unstable regions. Most of the asteroids have inclination smaller than 30 degrees (Fig.8).
Fig.7. Eccentricities of main belt asteroids
Fig.8. Inclinations of main belt asteroids
The inclination of long-period comets range uniformly over 0-180 degrees given that the semi major axis is larger than 100 au. (Fig.9)
Fig.9. Inclinations of main belt asteroids
2.3 Formation and transportation
Arika discussed formation and transportation of Oort cloud. After the formation of planets, there were still many planetesimals. Given planetary perturbation, their orbits became very elongated. The aphelion distances were also increased, and some of their aphelion distances became more than 1000 au. Perturbations from the Galaxy and stars changed the spherical distribution and randomized the inclinations.
Fig.10. Formation of the Oort cloud
Planetary scattering powered the outward transportation. A small body has a close encounter to a planet, is approximated as a 2-body problem.The orbit of the small body with respect to the planet is hyperbolic (Fig.11). The absolute value of small body will not change before and after the encounter, but the direction will change.
Fig.11. 2-body approximation region
If the sun is in the center of planetary orbit, then the orbit of the small body with respect to the Sun may drastically change. If only the small body earns the velocity change, it may go toward the Oort cloud (figure 12). This gives us inspiration that if we design the orbit of the spacecraft to make it approach a planet as, it will gain a velocity change .. In this way a "fly-by” goal which is often the mission design can save energy.
Fig.12. Planetary perturbation makes the orbits very elongated
Orbits of bodies with large aphelion distances are affected by external forces, perturbations from the galaxy and stars. To survive on elliptic orbits around the sun, it is crucial to increase the perihelion distances.
Fig.13. Perihelion distances
If perihelion distances stay close to a planet, the bodies will be eventually ejected from the solar system by the perturbation from the planets. External forces also change the inclinations. The structure of a swarm of small bodies becomes spherical, and that is how the Oort cloud appeared. Planets and other star systems gave a hand to the formation of the Oort cloud.
Fig.14. Group Photo of session 28
In the end, Arika answered questions from students and talked about her future research direction.