Did you know that both Venus and Neptune have orbits around the Sun that are more circular than the orbit of the Earth? In fact, Venus has the most circular orbit of all the major planets in the solar system. Here are the current orbital eccentricities of these three planets.
| Venus | 0.007 |
| Neptune | 0.010 |
| Earth | 0.017 |
Perhaps surprisingly, there are currently 1,959 minor planets with orbits more circular than that of Venus. There may be more, but I’ve excluded any minor planets where the 1σ uncertainty in eccentricity is greater than the eccentricity itself. Large uncertainties happen because some minor planets do not yet have well-determined orbits.
The most circular orbit of all minor planets with a well-determined orbit is 232812 (2004 RG298). It has not yet received a name. It is a main-belt asteroid and has an amazingly circular eccentricity (e) of 0.0000152412. It orbits at a distance of 2.37 AU from the Sun once every 3.65 years. Its orbit is inclined 2.4° to the ecliptic. Its physical size is unknown.
Among the named minor planets, the one with the most circular orbit is 12083 Darone. It is also a main-belt asteroid and has an eccentricity of 0.00177265. It orbits at a distance of 2.13 AU from the Sun once every 3.11 years. Its orbit is inclined 2.6° to the ecliptic. 12083 Darone has a diameter between 3.5 and 3.8 km. The lucky person to have this asteroid named after them is Gregory Darone, who mentored a finalist in the 2019 Regeneron Science Talent Search, a science competition for high school seniors. He teaches chemistry at the Charter School of Wilmington, Wilmington, Delaware.
The two largest trans-Neptunian objects are Pluto and Eris—both with high orbital eccentricities—and among the major planets, Mercury has the highest orbital eccentricity. Here are their current values.
| Eris | 0.438 |
| Pluto | 0.245 |
| Mercury | 0.206 |
Eris has the highest orbital eccentricity of these three objects by far. Eris orbits between 38.2 and 97.7 AU from the Sun. Its current distance is 95.5 AU and it is very gradually getting closer to the Sun. At times during their respective orbits, Eris can be closer to the Sun than Pluto. Eris orbits the Sun once every 559.9 years at an orbital inclination of 43.9° with respect to the ecliptic.
Pluto orbits between 29.6 and 48.7 AU from the Sun. Its current distance is 35.6 AU and it is gradually getting further from the Sun. During the years 1979 to 1999, Pluto was closer to the Sun that Neptune, and this will next happen again from 2223 to 2243 (approximately). Pluto orbits the Sun once every 244.7 years at an orbital inclination of 17.0° with respect to the ecliptic.
Mercury orbits between 0.31 and 0.47 AU from the Sun. Mercury orbits the Sun once every 88 days at an orbital inclination of 7.0° with respect to the ecliptic.
All three of these objects are smaller than Jupiter’s moon Ganymede!
Incidentally, there is no known minor planet that orbits completely inside the orbit of Mercury, but (as of this writing) 523 asteroids and 1,608 comets have come closer to the Sun than Mercury ever does. Sometimes a comet comes so close to the Sun that it disintegrates completely. Actual collisions with the Sun do occur.
Getting back to our original topic of nearly-circular orbits, the Earth’s orbital eccentricity has ranged from as circular as 0.000055 to as elliptical as 0.0679 due to gravitational perturbations caused by all the other objects in our solar system. This variation in orbital eccentricity is one of the Milankovitch cycles. Currently, the Earth’s orbital eccentricity is becoming more circular.
The orbits of all objects in our solar system change with time (albeit usually gradually and by a gentle amount). Over millions or billions of years, the gravitational attraction from stars passing close to the solar system can nudge orbits, too. Our solar system is a much more dynamic place than anyone imagined a century ago. Barring the extremely unlikely event of a star (lots of advance warning) or a rogue black hole (much less advance warning) entering our solar system, the Earth’s orbital eccentricity should remain stable for a very long time to come.