SaltWire E-Edition

Could Earth be demoted to dwarf planet status like Pluto?

GLENN ROBERTS glennkroberts@gmail.com @chronicleherald

Is Earth a dwarf planet? As absurd as it may seem, considering this question does warrant some investigation. The International Astronomical Union (IAU) is the international authority responsible for naming and classifying all celestial objects. It categorizes a "planet" as a celestial body that orbits the sun, has enough mass for gravity to create a spherical shape, and does not share its orbit with any other space object.

A "dwarf planet," according to the IAU definition, meets the first two criteria, but does not meet the last — it fails to clear its orbital neighbourhood of other objects.

On the surface, this would appear straightforward and a reasonable criterion. It was this definition that resulted in Pluto, formerly our solar system's ninth planet, being stripped of its planet status and demoted to that of a dwarf planet back in 2006.

This was primarily due to the discovery in 2005 of Eris, a trans-Neptunian (past Neptune's orbit) object of a similar size to Pluto (Pluto has a diametre of 2,377 kilometres; Eris's diametre is 2,326 kms). Interestingly, Eris was first thought to be our solar system's 10th planet.

As of March 2020, there were five official IAU-designated dwarf planets: Pluto, Eris, Makemake, Haumea, and Ceres. Four others — Quaoar, Orcus, Sedna, and Gonggong — are under consideration for official designation as dwarf planets.

It should be noted that not all planetary scientists agree with the IAU's definition of a dwarf planet, and many do not agree with Pluto's demotion. Almost all the recognized dwarf planets in our solar system, including Pluto (except Ceres, which lies in the asteroid belt between Mars and Jupiter), orbit within or just on the outer edge of the Kuiper Belt, a dense region of small, icy objects far beyond Neptune. Future investigations of this region could well result in the discovery of numerous

other dwarf planets.

WHAT ABOUT EARTH?

Returning to the question of Earth as a dwarf planet, on April 27, 2016, astronomers discovered Asteroid 2016 H03. This very small asteroid is just between 40 and 100 metres in size and is a member of the Apollo Group of asteroids, the largest group (with 10,486 known members) of near-Earth objects (NEOs) that follow Earth in its orbit around the sun, some of which periodically intercept our planet's orbit.

H03 is the smallest, closest, and most stable of these objects — its orbit has been stable for at least a century and is expected to remain so for several more centuries.

Because it circles Earth, albeit in a very erratic orbit, and is too distant to be considered a true satellite — it never ventures more than 100 AUs (astronomical units; these equals approximately 150 million kms) — it is classified as a quasi-satellite of Earth.

Herein lies the problem, at least in respect to the IAU's definition of a dwarf planet. If Earth has not cleared its orbit of all other space objects, in this case Asteroid H03, should it not then be classified as a dwarf planet rather than a planet?

Some planetary scientists suggest doing away with the dwarf planet classification and defining a planet, regardless of its orbital parameters, as a celestial body with sub-stellar mass, has never undergone nuclear fusion, and has enough self-gravity to assume a spherical or near-spherical shape.

While this definition would keep Earth and Pluto classified as planets, it would also include various planetary moons and currentlydesignated dwarf planets as planets.

Clearly, as astronomers discover other celestial bodies orbiting our sun and distant stars, what should be designated a planet or a dwarf planet will remain a topic of much discussion.

THIS WEEK’S SKY

Mercury (magnitude -0.7, look for it in Virgo — the Virgin), now visible in the

dawn sky, rises around 6 a.m. ADT (6:30 NDT), and reaches 12 degrees above the eastern horizon before fading from view by about 7:30 a.m. ADT (8 a.m. NDT). On Oct. 25, Mercury reaches its greatest western elongation (angular distance from the sun as seen from Earth), and on Oct. 26, reaches its highest altitude (17 degrees) above the eastern

horizon. Look for Mercury just above the eastern horizon directly below the waning, crescent moon about an hour before sunrise on Oct. 30.

Venus (magnitude -4.4, look for it in Ophiuchus — the Serpent Bearer) becomes visible around 6:30 p.m. ADT (7 p.m. NDT), 10 degrees above the southwest horizon, before sinking towards the horizon and setting around 8:05 p.m. ADT (8:35 p.m. NDT). On Oct. 29, Venus is at its greatest eastern elongation.

Mars, too close to the sun, remains unobservable.

Jupiter (mag. -2.5, visible in Capricornus — the Sea Goat) is visible 22 degrees above the southeast horizon around 6:30 p.m. ADT (7 p.m. NDT), reaching an altitude of 28 degrees above the southern horizon by 8:35 p.m. ADT (9:05 p.m.NDT), and remaining visible until about 12:35 a.m. ADT (1:05 a.m. NDT), when it sinks below seven degrees above the southwest horizon.

A much dimmer Saturn (magnitude +0.6, look for it in Capricornus) is visible around

6:45 p.m. ADT (7:15 p.m. NDT), 23 degrees above the southern horizon, remaining visible until about 10:50 p.m. ADT (11:20 p.m. NDT) when it drops below 11 degrees above the southwest horizon.

Until next week, clear skies.

EVENTS

Oct. 25 — Mercury at greatest elongation west

Oct. 26 — Mercury at highest morning altitude

Oct. 28 — Last-quarter moon Oct. 29 — Venus at greatest elongation east

Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. He welcomes comments from readers at glennkroberts@gmail.com.

SCIENCE

en-ca

2021-10-26T07:00:00.0000000Z

2021-10-26T07:00:00.0000000Z

https://saltwire.pressreader.com/article/281818582039312

SaltWire Network