A charming letter from 6-year old Cara O’Connor to NASA has delightfully brought back the debate over the status of Pluto — is it a Planet or is it a Dwarf Planet?
Cara O’Connor is a six-year-old student of Glasheen Girls’ School in Glasheen, Co Cork, Ireland. Her letter, written in April 2017, is reproduced in its entirety below (source www.washingtonpost.com/...).
Cara obviously knows her astronomy and wants to be an astronaut when she grows up. And her request is quite direct and emphatic —
I would really like it if everyone at NASA could please change your minds and make Pluto a planet again.
I am hoping that one day I can become an astronaut and work for ye at NASA but you need to fix this problem for me.
After several months, she got a reply from Dr Carly Howett, a senior scientist in the New Horizons spacecraft team — who concurs with Cara’s sentiment, even if technically Pluto is better classified as a dwarf planet -
Dr. James Green, the director of Nasa’s Planetary Science Division responded to Cara in the letter shown below. However, Dr Green was non-committal — “To me, it’s not so much about whether Pluto is a dwarf planet or not: it’s that Pluto is a fascinating place that we need to continue to study.”
Both letters are very appreciative of Cara’s effort and both are very encouraging of Cara’s dream to grow up to be an astronaut.
The Redefinition of Pluto
You can read about the background on how and why Pluto was reclassified as a Dwarf Planet over 12 years ago and efforts to restore Pluto’s status in the diary “Pluto - Planet or Dwarf Planet?” at www.dailykos.com/…
Here is a short summary — in 2006 the governing body of astronomy, the International Astronomical Union (IAU), not NASA, downgraded Pluto from a planet to a dwarf planet.
The definition of planet set in Prague, Czech Republic in August 2006 by the IAU states that, in the Solar System, a planet is a celestial body which:
- is in orbit around the Sun,
- has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
- has "cleared the neighborhood" around its orbit.
A non-satellite body fulfilling only the first two of these criteria is classified as a "dwarf planet".
Pluto fails to meet the third condition, because its mass is only 0.07 times that of the mass of the other objects in its orbit (part of the Kuiper belt); Earth's mass, by contrast, is 1.7 million times the remaining mass in its own orbit. Hence, it was re-classified as a dwarf planet.
Alan Stern, principal investigator of the New Horizons mission to Pluto, and others has been opposed to this definition and have proposed a different definition of "planet". The new definition will restore planetary status to Pluto.
A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.
In simpler words, planets are “round objects in space that are smaller than stars”.
The new definition classifies dwarf planets and moon planets such as Ceres, Pluto, Charon, and Earth’s Moon as “full fledged” planets. With this definition of a planet, the Solar System contains at least 110 known planets.
The author’s address the obvious concern of naming 110 planets as follows -
Certainly 110 planets is more than students should be expected to memorize, and indeed they ought not. Instead, students should learn only a few (9? 12? 25?) planets of interest. For analogy, there are 88 official constellations and ~94 naturally occurring elements, yet most people are content to learn only a few. So it should be with planets.
Dwarf Planets
The IAU currently recognizes five dwarf planets - Ceres, Pluto, Haumea, Makemake and Eris.
Another hundred or so known objects in the Solar System are suspected to be dwarf planets. Estimates are that up to 200 dwarf planets will be identified when the entire region known as the Kuiper belt is explored, and that the number may exceed 10,000 when objects scattered outside the Kuiper belt are considered.
Epilogue
This story has only recently gotten attention, although it took place last year. Let’s hope that this conversation kick-starts the debate and bears some fruit that satisfies proponents of both sides of the argument. And may the dreams of Cara and other kids like her come true.
Further Reading
- Pluto - Planet or Dwarf Planet? — www.dailykos.com/…
- “A Geophysical Planet Definition”, Stern at al, www.hou.usra.edu/...
- IAU definition of planet — en.wikipedia.org/…
- Pluto and the Developing Landscape of Our Solar System (IAU 2006 paper) — www.iau.org/…
- “How I Killed Pluto and Why It Had It Coming”, Mike Brown — www.amazon.com/...