WEBVTT 00:01.167 --> 00:03.436 position:10% line:85% size:80% Joan Cartan-Hansen, Host: From up here, you can see all the 00:03.436 --> 00:06.239 position:10% line:85% size:80% planets in our solar system, but do you know exactly what a 00:06.239 --> 00:07.774 position:10% line:85% size:80% planet is? 00:07.774 --> 00:08.942 position:10% line:85% size:80% Find out. 00:09.609 --> 00:15.382 position:10% line:85% size:80% (MUSIC) 00:15.382 --> 00:18.585 position:10% line:85% size:80% Mercury, Venus, Mars, Jupiter and Saturn were the first 00:18.585 --> 00:20.553 position:10% line:85% size:80% space objects called planets. 00:20.553 --> 00:24.657 position:10% line:85% size:80% The word planet means wanderer because the ancient Greeks saw 00:24.657 --> 00:27.093 position:10% line:85% size:80% them as moving lights in the sky. 00:27.093 --> 00:29.829 position:10% line:85% size:80% But the ancient Greeks didn't think of Earth as a planet. 00:29.829 --> 00:32.532 position:10% line:85% size:80% They thought it was the center of the universe and all space 00:32.532 --> 00:34.200 position:10% line:85% size:80% bodies revolved around it. 00:34.200 --> 00:37.337 position:10% line:85% size:80% Over time, humans learned the Sun is the center of our solar 00:37.337 --> 00:38.605 position:10% line:85% size:80% system. 00:38.605 --> 00:41.474 position:10% line:85% size:80% And all the planets, including Earth, orbit around it. 00:41.474 --> 00:44.110 position:10% line:85% size:80% Still, defining a planet was back then was easy. 00:44.110 --> 00:46.980 position:10% line:85% size:80% Everything, the Sun, the Moon, the Earth, all space bodies 00:46.980 --> 00:48.448 position:10% line:85% size:80% were called planets. 00:48.448 --> 00:50.817 position:10% line:85% size:80% Until a new invention came along. 00:50.817 --> 00:54.120 position:10% line:85% size:80% In 1781, Uranus became the first planet discovered by 00:54.120 --> 00:55.321 position:10% line:85% size:80% telescope. 00:55.321 --> 00:57.757 position:10% line:85% size:80% And by then, scientists had realized that the Sun was a 00:57.757 --> 01:00.126 position:10% line:85% size:80% star and the Moon was a natural satellite of the 01:00.126 --> 01:01.127 position:10% line:85% size:80% Earth. 01:01.127 --> 01:04.030 position:10% line:85% size:80% But everything else was pretty much described as a planet. 01:04.030 --> 01:08.301 position:10% line:85% size:80% Neptune was discovered in 1846 and Pluto in 1930. 01:08.301 --> 01:11.237 position:10% line:85% size:80% But Pluto is different from terrestrial planets like 01:11.237 --> 01:14.974 position:10% line:85% size:80% Mercury, Venus, Earth and Mars or the gas giants like Jupiter 01:14.974 --> 01:19.112 position:10% line:85% size:80% and Saturn or the ice giants like Uranus or Neptune. 01:19.112 --> 01:20.380 position:10% line:85% size:80% Pluto is small. 01:20.380 --> 01:23.049 position:10% line:85% size:80% And its moon, Charon, is half the size of Pluto. 01:23.049 --> 01:25.785 position:10% line:85% size:80% It's so big that the two are sometimes called a double 01:25.785 --> 01:27.387 position:10% line:85% size:80% planet system. 01:27.387 --> 01:30.523 position:10% line:85% size:80% Pluto has an usual orbit that sometimes brings it closer to 01:30.523 --> 01:31.891 position:10% line:85% size:80% the Sun than Neptune. 01:31.891 --> 01:34.761 position:10% line:85% size:80% And its orbit isn't circular like the other planets. 01:34.761 --> 01:38.131 position:10% line:85% size:80% Pluto is different and that lead to confusion. 01:38.131 --> 01:41.601 position:10% line:85% size:80% By 1992, planetary scientists found a number of ice worlds 01:41.601 --> 01:43.436 position:10% line:85% size:80% in the Kuiper Belt. 01:43.436 --> 01:44.304 position:10% line:85% size:80% Brian Jackson, Assoc. 01:44.304 --> 01:46.673 position:10% line:85% size:80% Prof. of Physics, Boise State University: The Kuiper Belt is 01:46.673 --> 01:49.142 position:10% line:85% size:80% a donut shaped region in the outer solar system, beyond the 01:49.142 --> 01:50.443 position:10% line:85% size:80% orbit of Neptune. 01:50.443 --> 01:53.646 position:10% line:85% size:80% And it's filled with objects, made up mostly of ice, so 01:53.646 --> 01:56.483 position:10% line:85% size:80% water ice, but also methane and ammonia ice. 01:56.483 --> 01:58.818 position:10% line:85% size:80% And some of these objects are large enough that we would 01:58.818 --> 01:59.819 position:10% line:85% size:80% call them ice worlds. 01:59.819 --> 02:02.489 position:10% line:85% size:80% And some of these ice worlds in the Kuiper Belt, they 02:02.489 --> 02:03.790 position:10% line:85% size:80% look a lot like Pluto. 02:03.790 --> 02:05.992 position:10% line:85% size:80% And so, astronomers were faced with this question. 02:05.992 --> 02:08.194 position:10% line:85% size:80% If we were going to call Pluto a planet, what were we going 02:08.194 --> 02:11.197 position:10% line:85% size:80% to call all these hundreds of thousands of other objects, 02:11.197 --> 02:13.867 position:10% line:85% size:80% very similar to Pluto in the Kuiper Belt. 02:13.867 --> 02:16.636 position:10% line:85% size:80% Cartan-Hansen: So, astronomers, decided they needed to clarify 02:16.636 --> 02:19.539 position:10% line:85% size:80% what exactly is a planet. 02:19.539 --> 02:23.176 position:10% line:85% size:80% In 2006, the International Astronomical Union, a 02:23.176 --> 02:26.012 position:10% line:85% size:80% worldwide group of top astronomers, came up with a 02:26.012 --> 02:28.114 position:10% line:85% size:80% new definition... 02:28.114 --> 02:30.817 position:10% line:85% size:80% A planet is a celestial body that is in orbit around the 02:30.817 --> 02:32.051 position:10% line:85% size:80% Sun. 02:32.051 --> 02:35.421 position:10% line:85% size:80% It has enough mass so gravity helps make it round. 02:35.421 --> 02:38.358 position:10% line:85% size:80% And it has cleared the neighborhood around its orbit. 02:38.358 --> 02:41.528 position:10% line:85% size:80% Jackson: So that means that the object has enough mass and 02:41.528 --> 02:45.231 position:10% line:85% size:80% enough gravity that it consumes or sweeps up other 02:45.231 --> 02:48.701 position:10% line:85% size:80% small bodies in its orbital path and Pluto, since there's 02:48.701 --> 02:51.337 position:10% line:85% size:80% a lot of other objects out there, doesn't do that. 02:51.337 --> 02:53.273 position:10% line:85% size:80% It's just too small. 02:53.273 --> 02:55.208 position:10% line:85% size:80% Cartan-Hansen: So, what to do with Pluto? 02:55.208 --> 02:58.044 position:10% line:85% size:80% Astronomers came up with the idea of a dwarf planet. 02:58.044 --> 03:00.480 position:10% line:85% size:80% A dwarf planet orbits around the Sun. 03:00.480 --> 03:03.383 position:10% line:85% size:80% It has enough mass and gravity to form an almost round shape. 03:03.383 --> 03:05.785 position:10% line:85% size:80% But it isn't big enough to clear the neighborhood. 03:05.785 --> 03:08.254 position:10% line:85% size:80% It also can't be a moon. 03:08.254 --> 03:11.791 position:10% line:85% size:80% As of 2020, we have five dwarf planets in our solar system. 03:11.791 --> 03:15.295 position:10% line:85% size:80% In order closest to the sun out, they are Ceres, Pluto, 03:15.295 --> 03:18.565 position:10% line:85% size:80% Haumea, Makemake and Eris. 03:18.565 --> 03:21.200 position:10% line:85% size:80% Ceres is the closest dwarf planet to the Sun. 03:21.200 --> 03:23.636 position:10% line:85% size:80% It's located in the asteroid belt, between Mars and 03:23.636 --> 03:26.406 position:10% line:85% size:80% Jupiter, making it the only dwarf planet in the inner 03:26.406 --> 03:27.907 position:10% line:85% size:80% solar system. 03:27.907 --> 03:30.376 position:10% line:85% size:80% Next comes Pluto, then Haumea. 03:30.376 --> 03:33.346 position:10% line:85% size:80% Haumea has an elongated shape rather than being totally 03:33.346 --> 03:36.449 position:10% line:85% size:80% round, probably because of the speed of its rotation. 03:36.449 --> 03:37.884 position:10% line:85% size:80% Next is Makemake. 03:37.884 --> 03:41.220 position:10% line:85% size:80% Makemake has a moon and is a classic Kuiper Belt object. 03:41.220 --> 03:42.689 position:10% line:85% size:80% And finally, Eris... 03:42.689 --> 03:45.625 position:10% line:85% size:80% Eris is almost as large as Pluto and was once considered 03:45.625 --> 03:48.661 position:10% line:85% size:80% our solar system's tenth planet. 03:48.661 --> 03:51.064 position:10% line:85% size:80% Jackson: Planetary scientists are still looking for dwarf 03:51.064 --> 03:52.165 position:10% line:85% size:80% planets. 03:52.165 --> 03:54.167 position:10% line:85% size:80% This is an active area of research. 03:54.167 --> 03:56.636 position:10% line:85% size:80% There may be another hundred in our solar system and 03:56.636 --> 04:00.506 position:10% line:85% size:80% probably hundreds more just outside of the Kuiper Belt. 04:00.506 --> 04:02.775 position:10% line:85% size:80% Cartan-Hansen: Not everyone agrees with the new definition 04:02.775 --> 04:04.010 position:10% line:85% size:80% of a planet. 04:04.010 --> 04:07.280 position:10% line:85% size:80% Some still think Pluto deserves that title. 04:07.280 --> 04:10.116 position:10% line:85% size:80% Jackson: That's true, but the definition of a planet may 04:10.116 --> 04:12.952 position:10% line:85% size:80% still change as we learn more about our solar system and 04:12.952 --> 04:16.389 position:10% line:85% size:80% just because Pluto and Ceres and all those other objects 04:16.389 --> 04:18.691 position:10% line:85% size:80% are called dwarf planets it really doesn't make them any 04:18.691 --> 04:19.993 position:10% line:85% size:80% less interesting. 04:19.993 --> 04:22.996 position:10% line:85% size:80% What we call them is almost the least interesting aspect 04:22.996 --> 04:25.598 position:10% line:85% size:80% of these objects which have a fascinating variety of 04:25.598 --> 04:26.699 position:10% line:85% size:80% phenomena. 04:26.699 --> 04:29.802 position:10% line:85% size:80% It's just our current way of classifying one of the many 04:29.802 --> 04:31.537 position:10% line:85% size:80% wonders of space. 04:32.372 --> 04:34.741 position:10% line:85% size:80% Cartan-Hansen: If you want to learn more about planets and 04:34.741 --> 04:37.010 position:10% line:85% size:80% dwarf planets, check out the science trek website. 04:37.010 --> 04:39.545 position:10% line:85% size:80% You'll find it at science trek dot org. 04:40.580 --> 05:00.433 position:10% line:85% size:80% (MUSIC) 05:00.433 --> 05:02.802 position:10% line:85% size:80% Announcer: Presentation of Science Trek on Idaho Public 05:02.802 --> 05:06.305 position:10% line:85% size:80% Television is made possible through the generous support 05:06.305 --> 05:09.375 position:10% line:85% size:80% of the Laura Moore Cunningham Foundation, committed to 05:09.375 --> 05:12.145 position:10% line:85% size:80% fulfilling the Moore and Bettis family legacy of 05:12.145 --> 05:14.614 position:10% line:85% size:80% building the great state of Idaho. 05:14.614 --> 05:17.917 position:10% line:85% size:80% By the Idaho National Laboratory, mentoring talent 05:17.917 --> 05:20.887 position:10% line:85% size:80% and finding solutions for energy and security 05:20.887 --> 05:24.891 position:10% line:85% size:80% challenges, by the Friends of Idaho Public Television and 05:24.891 --> 05:27.226 position:10% line:85% size:80% the Corporation for Public Broadcasting.