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Discovery of a New Planet Leaves The Solar System With Only 8 Planets

Hey... how many planets are there in our Solar System? The answer is 8! Wondering? Read out...

Until 2006, nothing was officially known about Eris. Eris is the largest known dwarf planet that had been discovered while a team of three astronomers, Mike Brown (Caltech), Chad Trujillo (Gemini Observatory), and David Rabinowitz (Yale University) were on a survey at Palomar Observatory's Samuel Oschin telescope. They suggested the name officially on September 6, 2006 which was accepted and announced on September 13, the same year. This discovery of new planet stirred up a buzz across Internet as well as many science magazines.

Myth About Its Name:

The name has been given in honor of the Greek goddess warfare and strife - ERIS, who's believe to breed the sense of jealousy and causes aggression among men. Greek mythology documents a story of the wedding of Peleus and Thetis, where all Gods were invited except Eris. Enraged at the exclusion, Eris spitefully caused a situation when Gods got involved in a quarrel that caused the Trojan war. Keeping the myth in mind, the researchers named the new planet Eris, as it caused a great lot of confusion among the members of international astronomical community in a discussion board at International Astronomical Union (IAU) in Prague, regarding the planet's proper designation. The meeting ended up with a conclusion that Pluto and Eris had to be demoted to Dwarf-Planet category, setting apart the solar system with only 8 planets! Both Pluto and Eris are the distant objects ever seen in orbit around the sun.

It was literally not possible to say how bit Eris was when it was first discovered. Researchers were gripped in confusions regarding it's size, because all they could see was a dot of light, which was actually the sunlight reflected off Eris's surface. Another big confusion was that the researchers didn't know if the object was bright because of it's highly reflective surface or because of it's large size or both. The discoverers said: "When an object is too far away to directly see how big it is, astronomers use an indirect method instead where they measure the heat coming from the object. If we wanted to measure the size of a fire, for example, we could do it by measuring the total amount of heat coming from the fire. The temperature of the flames in a match and a bonfire are essentially the same, but a bonfire emits much more heat because it is much bigger. The same is true of distant planets. Because we know how far away the planet is we have a pretty good idea of the surface temperature (a frosty 405 degrees below zero!), thus when we measure the total heat we can tell how big the object is. Unfortunately, the new planet is so far away and so cold that our first attempt at measuring the heat, using the Spitzer Space Telescope, could not detect the heat output. This fact tells us that the object must be smaller than about 3300 km."

Discovery of a New Planet Solar System 8 planets about Eris dwarf planet dwarf-planet surface of Eris distance of Eris from the sun Astral Science,  Planet Science
Atmosphere and Surface of Eris

The discoverers study the composition of the dwarf-planet, Eris by studying the sunlight reflected off of the new object. They used the Gemini Observatory on Mauna Kea, Hawaii to study sunlight reflected off of the surface of Eris and found remarkable similarities with Pluto, although not completely identical! Surface of Pluto is moderately red, while Eris appears to be white. Since Pluto has a mottled surface, it reflects 60% of the sunlight on an average, which hits it. Eris has comparatively uniform surface, which makes 86% (+/- 7%) of the sunlight to reflect back. The distance of Eris from the sun is so great that its surface is a frozen solid! In around 280 years or so, Eris will be around 2.6 time closer than it's current location from the sun. Over the next 280 years the absolute temperature of Eris will eventually rise by a factor of 1.6 (which is the square root of 2.6). Currently the temperature of Eris is minus 405 degree.

Facts About This New Planet - Eris:
  • Discoverers - Brown, Trujillo, Rabinowitz
  • Size - 2400 +/- 100 km (105% Pluto)
  • Brightness - 4th brightest Kuiper belt object (KBO)
  • Current distance - 97 AU
  • Orbital period - 560 years
  • Closest approach to sun - 38 AU
  • Visibility - late summer, fall, early winter
  • Tilt of orbit - 44 degrees
  • Furthest from sun - 97 AU

Note: AU means Astronomical Units. 1 AU = 149 598 000 kilometers


Laurel Kornfeld said…
The answer to the number of planets in our solar system is not eight--it is still in question, with many currently counting 13. Only four percent of the IAU voted on the demotion of Pluto, and most were not planetary scientists. Their decision was immediately rejected by hundreds of professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA's New Horizons mission to Pluto. Stern and like minded scientists rejected the criterion that an object has to "clear its orbit" to be a planet. They prefer a broader planet definition that includes any non-self-luminous spheroidal body orbiting a star. That gives us 13 planets: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, and Eris. Interestingly, Eris' co-discoverer, Dr. David Rabinowitz, signed the petition rejecting the IAU planet definition and the demotion of Pluto.

The IAU definition makes no sense in that it states dwarf planets are not planets. That is like saying a grizzly bear is not a bear. It is also contrary to the use of the word "dwarf" in astronomy, where dwarf stars are still stars, and dwarf galaxies are still galaxies. Also, the IAU definition classifies objects solely by where they are while ignoring what they are. If Earth were in Pluto's orbit, according to the IAU definition, it would not be a planet either.

Visit my Pluto blog at for more on worldwide efforts to overturn the controversial demotion. This debate is far from over.