Wednesday, February 18, 2009

Mars, Escaping Earth, and Why Pluto isn't a Planet

Jennifer wrote:
Dear Planetary Astronomer Mike,

I have a 5 year old son who's heart is set on traveling to mars (and a three year old daughter who would like to rocket ship to mars). He often has questions about the planets, the sun, and earth.
Well, we'll see what happens to Bush's lofty goal of getting humans to Mars. It's not such a terrible idea, just completely underfunded by the last administration.

Most planetary astronomers I know are waiting with baited breath to see who Obama appoints as NASA administrator, as it will deeply affect future science policy. From reading the tea leaves, the guess is that there will a lot of new funding for Earth climate satellites and research...which also isn't such a terrible idea.

Jennifer continued:
My daughter (who is determined to be in this email because she must be just like her brother) wants to know how fast a rocket ship has to move to get her to the moon.
Seven miles per second. That's the Earth's escape velocity, and the energy required to escape its gravity well...any slower and you'll fall back to Earth.

Jennifer concluded with:
For the last week my son's been asking me why Pluto is not a planet and what it is/how it formed.
Okay, this is actually a pretty popular question, and I've given a couple talks about it. It's also a question that that hits home - our astronomy department was founded by Clyde Tombaugh, discoverer of Pluto.

So, imagine the following scenario:
  1. Astronomers have mapped out our solar system, and after some analysis are expecting to find a planet where one has not yet been found.

  2. After careful searching and some serendipity, an observer finds a tiny light moving among the heavens. Plotting its course, it turns out to be an object exactly where a planet was expected...though it's somewhat smaller than expected.

  3. Astronomers rejoice! A new planet has been found! We're so smart!

  4. Time passes...when suddenly a new object is discovered at almost the exact same distance from the Sun...curious. Another planet? In the same orbit?

  5. Another object at the same distance is discovered.

  6. Another object at the same distance is discovered.

  7. Another object at the same distance is discovered.

  8. etc.

  9. After compiling a vast array of objects all orbiting in roughly the same orbit, maybe these aren't planets after all.
The above scenario perfectly outlines what actually happened...with Ceres, the first asteroid ever discovered. (Ha! Tricked you!)

By the late 1700's, astronomers were disturbed by the conspicuous gap between Mars at 1.6 AU and Jupiter at 5.2 AU (1 astronomical unit = 1 AU = the distance between Earth and the Sun). Titius and Bode drew up an entirely empirical equation to calculate the distance of planets from the Sun which worked out quite nicely. There was a missing term in their equations, though...precisely in this gap, around 2.5 AU, suggesting there should be a planet there. Quoting Titius from 1768:
"After Mars there follows a space of 4+24=28 parts, in which no planet has yet been seen. Can one believe that the Founder of the universe had left this space empty? Certainly not."
The stage was further set by Herschel, who had discovered the planet Uranus in 1781. After his discovery, astronomers had warmed to the idea of finding new planets was new, it was happening, it was the in thing to do. Moreover, Uranus fits perfectly as the next term of the Titius-Bode equation. A coordinated observing campaign was begun to search for the missing planet.

Along comes Giuseppe Piazzi, an Italian monk and hobby astronomer. After dutifully scanning the heavens, on the first night of the new millennium, January 1st, 1801, he observes a tiny "star" where none had been before. He follows it night-to-night and observes it first it was thought to be a comet, but after careful calculation astronomers realize this is exactly what they've been looking for. It is dubbed Planet Ceres. All is well, and astronomers pat themselves on the back for being so very clever.

Then in 1802, Heinrich Olbers finds another object at roughly the same distance as Ceres...the, uh, Planet Pallas! In 1804, Planet Juno is 1807, Planet Vesta is is discovered...something is amiss. By 1850, there were 13 of these new planets...Here's a page from the 1850 Annual of Scientific Discovery which documents all 18 planets at the time.

It wasn't until 1852 that these objects were reclassified as "minor planets"...which is a good thing, since we know now of roughly 300,000 minor planets that are hardly on the same footing as the classic 8. Science-wise, it makes a lot more sense to group these objects into a family of astronomical bodies, the asteroid belt. Still, for 51 years Ceres enjoyed full planet status.

Now, also in this time period (1846) Neptune was discovered at 30 AU. Small gravitational perturbations in the motion of Uranus caused scientists to theorize the existence of a large planet further out. Le Verrier and Adams both independently calculated where such a planet should be...but Le Verrier was given telescope time first, and Neptune was found very close to its predicted position. This was a huge triumph of predictive science (and one of the few cases in history where the scientific method was more important to discovery than serendipity...but that's another topic).

Hoping to follow on this success, Percival Lowell hoped to do something similar some 80 years later. Having then detected subtle perturbations in the orbit of Neptune, he funded the search for "planet X", a large planet even further out. Initial observations were less than successful...planet X's position had to be recalculated 3 times after initial searches turned up nothing. But, in 1930, Clyde Tombaugh observing at Lowell Observatory discovered a tiny light moving night-to-night. 40 AU out...this was it! Planet Pluto! Once again, astronomers congratulated themselves on their sheer brilliance, and their fancy scientific method.

At first, Pluto was calculated to have a mass 7 times larger than the Earth. Over time, though, this was amended, and it turned out the new planet was a bit smaller. A lot smaller, in fact. In 1978, Pluto was found to have a moon, which allowed a precise determination of its mass...Pluto was found to be about 5 times less massive than our Moon. But, hey, it's a planet, who are we to argue?

By 1989, the scientific method took another blow...the Voyager spacecraft passed by Neptune allowing for a precise orbit determination, and discovered that those initial gravitational perturbations which Lowell measured simply didn't exist at all. They were observational errors. Planet X was found, again, by sheer serendipity...the third predicted position based on flawed Neptune data just happened to line up with the position of an actual object.

Still a planet, though, right? Things get trickier in 1992...David Jewitt discovers 1992 QB1 orbiting at almost the same distance as Pluto from the Sun. It's only one-fifth the radius of Pluto, though, so that's hardly planet-y at all.

Time passes. More objects are discovered around this distance from the Sun...about 1,000 more, though they're all smaller than Pluto. This situation is starting to sound really familiar, when suddenly in 2006 Mike Brown discovers 2003 UB 313 (now known as Eris). Radius determinations find that Eris is actually larger than Pluto...uh oh.

Crisis ensues. If Pluto is a planet, and Eris is bigger, then Eris is a planet, too, right? Or was Pluto never really a planet, after all? The International Astronomical Union, the body responsible for all solar system object naming, steps in. It's put to a vote, and the criteria for a planet are established:
  • It has to orbit a star. (Pluto: check.)
  • It has to have enough mass to be round. (Pluto: check.)
Initially this was would have included Pluto as a planet, but 52 other solar system objects as well. That's a few too many to be useful, so, one more criterion was added:
  • It must have cleared its orbit of all other bodies. (Pluto: crap.)
That final criterion is the problem for Pluto. It's a simple calculation: just compare the mass of a planet to the mass of all the stuff in the planet's orbit. Earth is responsible for 99.999% of all the mass of in its orbit. Pluto, on the other hand, is responsible for just 7% of the mass in its orbit.

Objects meeting criteria 1 & 2 but not criterion 3 are now "dwarf planets"...officially this include Pluto, Eris, Haumea, Makemake, and Ceres so far. Pluto enjoyed full planetary status for 76 years...not terribly different than Ceres, really.

It also makes sense...just as all the objects between Mars and Jupiter are collectively known as the asteroid belt and have a common origin, so all the object around 40 AU are collectively known as the "Kuiper Belt" and have a common origin. This bring us to the final part of your question: what is it, and how did it form.

Until recently, there were two competing theories for solar system formation: gas instability and core accretion. Both start with the extremely early solar system, where a swirling disc of gas and dust surrounded the proto-star that would become our Sun. Gas instability says that little over-dense nuggets of gas and dust in the disc gravitationally collapse to make planets. Core accretion states that first dust particles stick together to start forming larger and larger planetesimals, whose gravity then grows to the point that they can start pulling in the surrounding gas. For several reasons I won't get into (unless someone asks), gas instability has fallen out of favor with the astronomical community, though the debate isn't quite over yet.

Based on core accretion, particularly in the outer solar system, you start with these planetary "seeds" that suck in gas and make gas giant some point, though, you run out of gas to accrete. Add to that the slow orbital speed and huge distances between objects out at 40 AU, and you'll find that seeds at Pluto's distance never merged together enough to get in on the big gas feast.

Anyway, we're pretty sure this is what happened to Pluto...put in terms for a 5-year-old, its big brothers Jupiter, Saturn, Uranus, and Neptune ate up all the gas before Pluto got any, so it's been left as just a lonely little planet seed. I think there's probably a moral in there somewhere for him about sharing with his sister, too.



  1. Olá sou de São José dos Campos São Paulo Brazil vim aqui fazer uma visita
    Abraços Antonio.

  2. This is amazingly interesting and understandable! Thanks for paring it down to the morale of the story. LOVE THE BLOG!