Tag: Eris

More Perfect Than Earth

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.

Venus0.007
Neptune0.010
Earth0.017

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.

To ensure that my restriction of σe < e wouldn’t be too severe, I confirmed that minor planet 232812 has the lowest known orbital eccentricity. The 11 minor planets that nominally have a lower eccentricity are all unnumbered, which means their orbits are not well determined and should thus be excluded.

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.

Eris0.438
Pluto0.245
Mercury0.206

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.

Eris: Plutoid, Dwarf Planet, or 10th Planet?

Eris was discovered on January 5, 2005 by Michael E. Brown, Chad Trujillo, and David A. Rabinowitz. Its orbit is more eccentric and more highly inclined than Pluto’s, and it is almost as large as Pluto, having a diameter that is 97.9% that of Pluto. Eris last came to perihelion on July 23, 1699 when it was in the constellation Virgo shining at a magnitude of 14.8, well beyond the reach of any telescopes existing at the time.

Pluto, Eris, and Satellites – Sizes and Orbital Distances to Scale

Eris has an orbit that is so eccentric (e = 0.44) that it actually spends some time each orbit closer to the Sun than Pluto is during the outer reaches of its orbit. Pluto’s aphelion distance is 49.31 AU, and Eris will be closer to the Sun than that for 99 years, from 2208 to 2307.

Eris is closer to the Sun than Pluto’s average distance of 40.70 AU for 43 years, between 2236 and 2279. Eris again reaches perihelion in 2257, when it will be 38.09 AU from the Sun.

Eris has an orbit that is tilted at nearly a 45° angle with respect to the ecliptic. This takes it through some interesting constellations during its 559-year orbital period. Here is its upcoming travel itinerary.

Upcoming Travel Plans for Eris (not subject to change1)

2022   Cetus
2036   Pisces
2059   Cetus
2064   Aries
2126   Perseus
2174   Camelopardalis
2197   Lynx
2208   Ursa Major
2237   Canes Venatici
2245   Coma Berenices
2256   Virgo
2274   Libra
2281   Hydra
2285   Centaurus
2286   Lupus
2298   Norma
2308   Ara
2320   Pavo
2357   Indus
2367   Tucana
2376   Grus
2399   Phoenix
2434   Sculptor
2487   Cetus

1 Unless the constellation boundaries are redrawn due to precession or other considerations

In Greek & Roman mythology, Eris is the goddess of strife and discord. 500 years hence, in 2522, Eris will once again be in Cetus, as it is today. But where will we be? What kind of life will our great-great-great-great great-great-great-great-great-great-great-great-great great-great grandchildren have in 2522? Here are some of my hopes for 2522.

  • Humanism will have replaced religion.
  • There will be no poverty in the world.
  • Everyone will have adequate health care, and it will be free.
  • Zero population growth will have been achieved by the only humane way possible: having fewer children.
  • There will be no more wars, no weapons of mass destruction.
  • There will be no need for guns, and no one will have them.
  • Violence will not be tolerated, nor will society glorify it or dwell on it in any way.
  • Individuals who “cross the line” and violate others through the use of physical violence will be psychologically re-engineered so they will live productive and fulfilling lives without being a threat to others. This neutralization of violent tendencies must be accomplished humanely and in a way that does not violate the individual’s essential humanity.
  • The Earth will be treated as the oasis it is.
  • Money will no longer exist, nor will it be needed.

Though no one alive today is likely to ever see any of these things, that in no way excuses us from working substantially towards these goals. To do anything less is a dereliction of moral duty.

A Case for Ten Planets

Clyde Tombaugh (1906-1997) spent the first fifteen years of his life on a farm near Streator, Illinois, and then his family moved to a farm near Burdett, Kansas (no wonder he got interested in astronomy!), and he went to high school there. Then, on February 18, 1930, Tombaugh, a self-taught amateur astronomer and telescope maker, discovered the ninth planet in our solar system, Pluto. It had been nearly 84 years since the eighth planet, Neptune, had been discovered, in 1846. And it would be another 62 years before another trans-Neptunian object (TNO) would be discovered.

Clyde Tombaugh made his discovery using a 13-inch f/5.3 photographic refractor at the Lowell Observatory in Flagstaff, Arizona.

Clyde Tombaugh was 24 years old when he discovered Pluto. He died in 1997 at the age of 90 (almost 91). I was very fortunate to meet Prof. Tombaugh at a lecture he gave at Iowa State University in 1990. At that lecture, he told a fascinating story about the discovery of Pluto, and I remember well his comment that he felt certain that no “tenth planet” larger than Pluto exists in our solar system, because of the thorough searches he and others had done since his discovery of Pluto. But, those searches were done before the CCD revolution, and just two years later, the first TNO outside the Pluto-Charon system, 15760 Albion (1992 QB1), would be discovered by David Jewitt (1958-) and Jane Luu (1963-), although only 1/9th the size of Pluto.

Pluto is, by far, the smallest of the nine planets. At only 2,377 km across, Pluto is only 2/3 the size of our Moon! Pluto has a large moon called Charon (pronounced SHAR-on) that is 1,212 km across (over half the size of Pluto), discovered in 1978 by James Christy (1938-). Two additional moons were discovered using the Hubble Space Telescope (HST) in 2005: Hydra (50.9 × 36.1 × 30.9 km) and Nix (49.8 × 33.2 × 31.1 km). A fourth moon was discovered using HST in 2011: Kerberos (10 × 9 × 9 km). And a fifth moon, again using HST, in 2012: Styx (16 × 9 × 8 km).

Pluto has been visited by a single spacecraft. New Horizons passed 12,472 km from Pluto and 28,858 km from Charon on July 14, 2015. Then, about 3½ years later, New Horizons passed 3,538 km from 486958 Arrokoth, on January 1, 2019.

Only one other TNO comparable in size to Pluto (or larger) is known to exist. 136199 Eris and its moon Dysnomia were discovered in 2005 by Mike Brown (1965-), Chad Trujillo (1973-), and David Rabinowitz (1960-). It is currently estimated that Eris is 97.9% the size of Pluto. Not surprisingly, in 2006 Pluto was “demoted” by the IAU from planethood to dwarf planet status. (Is not a “dwarf planet” a planet? Confusing…)

My take on this is that Pluto should be considered a planet along with Eris, of course. The definition of “planet” is really rather arbitrary, so given that Pluto was discovered 75 years before Eris, and 62 years before TNO #2, I think we should (in deference to the memory of Mr. Tombaugh, mostly) define a planet as any non-satellite object orbiting the Sun that is around the size of Pluto or larger. So, by my definition, there are currently ten known planets in our solar system. Is that really too many to keep track of?

There is precedent for including history in scientific naming decisions. William Herschel (1738-1822) is thought to have coined the term “planetary nebula” in the 1780s, and though we now know they have nothing to do with planets (unless their morphology is affected by orbiting planets), we still use the term “planetary nebula” to describe them today.

In the table below, you will find the eight “classical” planets, plus the five largest TNOs, all listed in order of descending size. (The largest asteroid, Ceres, is 939 km across, and is thus smaller than the smallest of these TNOs.)

You’ll see that the next largest TNO after Eris is Haumea, and that its diameter is only 67% that of Eris.

I’ve also listed the largest satellite for each of these objects. Venus and Mercury do not have a satellite—at least not at the present time.

It is amazing to note that both Ganymede and Titan are larger than the planet Mercury! And Ganymede, Titan, the Moon, and Triton are all larger than Pluto.

Largest Objects in the Solar System

Object Diameter (km) Largest Satellite Diameter (km) Size Ratio
Jupiter 139,822 Ganymede 5,268 3.8%
Saturn 116,464 Titan 5,149 4.4%
Uranus 50,724 Titania 1,577 3.1%
Neptune 49,244 Triton 2,707 5.5%
Earth 12,742 Moon 3,475 27.3%
Venus 12,104 N/A N/A N/A
Mars 6,779 Phobos 23 0.3%
Mercury 4,879 N/A N/A N/A
Pluto 2,377 Charon 1,212 51.0%
Eris 2,326 Dysnomia 700 30.1%
Haumea 1,560 Hiʻiaka 320 20.5%
Makemake 1,430 S/2015 (136472) 175 12.2%
Gonggong 1,230 Xiangliu 200 16.3%

Should any other non-satellite objects with a diameter of at least 2,000 km be discovered in our solar system, I think we should call them planets, too.