Starry Heavens Newsletter
April 2017

---

To be or not to be a planet
The debate over whether Pluto should be called a planet or not raged for years, but seemingly ended in 2006 when it was reclassified as a dwarf planet by the International Astronomical Union (IAU). The debate is being ignited once again by Alan Stern from the Southwest Research Institute in Boulder, Colorado.
 
Alan Stern led the groundbreaking New Horizons Mission to Pluto in 2015, and has co-authored a paper with Kirby Runyon calling for a reclassification of what a planet is. The paper proposes a new definition that would essentially make any large body in space, including a moon, a planet.
 
Stern tells IFLScience that they propose “There would be three criteria [for a planet]: it is a body in space; it is large enough to be rounded by its own gravity; it is not so large that it starts fusion in its interior.” If this definition is adopted, it would essentially increase the number of planets in our Solar System from eight at the moment to approximately 110. It would include the major planets, dwarf planets like Pluto and Ceres, and large satellites like Titan, Europa, and even our own Moon.
 
The current IAU definition of a planet also has three criteria. It must be a celestial body in orbit around the Sun. It must have sufficient mass for its self-gravity to make it round. It must have cleared its neighborhood in its orbit. This latter criteria led to the reclassification of Pluto. But it’s also  the most controversial aspect, with many pointing out that worlds like Jupiter and even Earth do not fulfill this criteria. Earth, for example, has many asteroids in its neighborhood that have not been cleared out, as does Jupiter.
 
The IAU definition fails to fully account for exoplanets, which are worlds beyond our Solar System, choosing to only classify major planets around our Sun. As we continue to discover more and more worlds of all shapes and sizes, this shortcoming becomes more apparent. Runyon explained to IFLScience “As more planets are discovered in the Kuiper Belt [at the edge of the Solar System] and around other stars, our interpretation of the discovered world has a different twist depending on whether we call it a planet or a 'non-planet dwarf planet,”
 
Instead, Runyon and Stern’s paper suggests that a planet be classified on its physical properties, rather than what’s going on in its vicinity. Large spherical bodies would be satellites, while smaller irregular objects would continue to be classified as small moons, asteroids, and comets. Stern argues “that the term moon or satellite is useful for describing location, but it doesn’t describe the type of object in orbit. When objects are large enough to be planets, and orbit other planets, we call them planets. Stars orbit stars. Galaxies orbit galaxies.
The fact that the universe puts planets around other planets is a fact of nature. You can’t really deny facts of nature.”
 
However, one gray area is brown dwarfs, large bodies that have not quite managed to ignite nuclear fusion in their cores, and thus fall on the border of giant planets and stars in both definitions. Future observations could help clarify exactly what they should be classified as.

                                **********************************
Seven Earth-sized planets: Trappist-1 System



NASA revealed that it has discovered 7 "Earth-sized" planets orbiting a nearby star that are relatively well positioned to support life.  The illustration above compares the newly discovered planets in the Trappist-1 System to our inner solar system.However, the planets are roughly 40 light-years away in the constellation Aquarius. Using existing technology it would take roughly 44 million years to get there. Nevertheless, it is exciting to find such a number of planets that scientists think approximate the conditions of earth.  Scientists say they need to study the atmospheres before determining whether these planets could support some type of life but at least three of them are in the so-called habitable zone, where water and, possibly life, might exist.

This discovery clarifies an announcement from last year of three Earth-sized planets orbiting the star known as Trappist-1...but over the last year the Dutch team of scientists led by Michael Gillon of the Universite’ de Liege discovered that what they thought was one planet is actually three separate planets. Two additional planets found in the Trappist-1 system bring the total to seven.
 
These planets are outside our solar system and are referred to as exoplanets. Exoplanets were first discovered and confirmed in the mid-1990s. Since then, some 3,500 exoplanets have been discovered. Given the billions of stars in the Milky Way galaxy, if each of them has at least one planet, that’s billions of possibilities for Earth-like planets.
                              

• $7 General admission
• $6 WCC students, military, seniors (65 years or older), with ID
• $5 Children (ages 4-12 years)
• Free for children under 4 years of age (1 per paying adult), and WCC faculty or staff with university ID

---

---