Archives For planetary science

pluto approach

According to some people, Pluto never stopped being a planet. While there was acrimony when the new definition was approved by the IAU, after a while it seemed that people got used to the idea that maybe, certain planet-like objects shouldn’t be called planets after all. However, as we approach Pluto with the fastest spacecraft ever built to study worlds like it, the person in charge of the mission’s science, Alan Stern, insists that it’s a planet and those who defined it otherwise lack a persuasive argument to call it anything else. According to him, if we start applying IAU’s definition to current planets, none would qualify because they can’t clear out their orbits and all have various stellar bodies crossing paths with them or following in their orbital wake. Jupiter is not even a proper planet because it attracts so many comets, Neptune can’t be a planet thanks to the fact the Pluto crosses its orbit, and Earth has a cloud of asteroid debris following it. And if none of these spheres is a planet, then what exactly is? But the catch here is that Stern may be emphasizing the letter of the definition over its spirit to score a rhetorical buzz-worthy point.

While he correctly says that a definition that could lead to hundreds of planets in our little solar system alone shouldn’t bother us because science is science and we need to call things as they are, rather than change definitions solely for the sake of convenience and textbook publication, how he interprets the requirement to clear one’s orbit is suspect. There’s math involved in how one determines if a planet cleared its orbital neighborhood and what is meant by cleared, and it should be pointed out that Stern co-authored a paper that contributed greatly to this concept in the first place some 15 years ago. Nowhere does it state that a planet must have a pristine orbit because such a thing is physically impossible in most solar systems. Instead, the idea is that it’s the dominant body in its orbit, and has enough scattering power to send incoming bodies away, which isn’t a perfect definition and could cause some semantic headaches in certain cases, but hardly as absolutist as Stern makes it sound. And the IAU debate raises a valid point. If we call anything round and orbiting a star a planet, how many planets would we have? At what point is there a difference significant enough between planets to require us to rethink the definition?

For what it’s worth, Stern does have an answer to that. Despite raging and fuming about how it all went down at the IAU meetings, he doesn’t want to get rid of the term dwarf planet. But in his mind, that’s just another type of a planet along with numerous other classifications he offered in his paper trying to define any planet’s orbital dominance. He sees us categorizing planets much like we do stars, from dwarfs to hyper-giants based primarily on mass, and each world falling at a certain point along a planetary Hertzsprung-Russell diagram. So what if we identify Ceres and Eris along with a whole host of Kupier Belt Objects as planets as long as they orbit the sun and have enough mass to become round? So what if we end up with 3,000 planets? Isn’t that better than arbitrarily drawing a cutoff at a number we can easily memorize solely for the purposes of nomenclature in classrooms? As we see with extrasolar systems, planets are weird things in all sorts of erratic orbits, so perhaps, how we define what is and isn’t a planet should reflect that in our literature. Plus imagine how big and colorful our model solar systems would get…

curiosity lander

Or at least that’s how Kanye West would’ve characterized the reaction of planetary scientists to NASA’s announcement that its next big mission in 2020 would be to send an updated Curiosity twin to Mars at a target cost of $1.5 billion. What’s the problem? Well, planetary science budgets aren’t exactly all that large or flexible so every dollar spent on Mars comes out of the budget of a future mission to Europa or Titan. And with NASA’s recent zeal about Mars, it seems like the red planet is squeezing out the rest of the solar system from the agency’s scientific priorities. Since everyone’s buzzing about Mars rovers, manned missions to Mars, potential cities on Mars, with a periodic misunderstanding about traces of microbial life on Mars thrown in for extra publicity, the visibility for missions beyond the cold, rusty desert world is plummeting and with it, the chance to get decent funding for an ambitious new mission deep into the outer solar system.

From a bureaucrat’s standpoint, you can see why NASA is eager to send more rovers to Mars. It worked out the kinks and really understands how to land robots on the red planet. Images being beamed by a rover from the surface of another world rocket across the web and TV, and prompt a thousand cheers for the agency, citing the latest landing as proof that NASA can still do truly amazing and awe-inspiring things, regardless of what the whiny curmudgeons think. But just like studio executives in Hollywood trying to sell the same movies again and again with new actors or new titles, NASA administrators could easily venture past the point of diminishing returns, when new rovers on Mars will produce little more than yawns and reruns of the same stories written as its predecessors touched down. The agency doesn’t have enough money or political capital to tie its future to Mars. In the 1980s, when it was still riding the Apollo high, it’s pricey proposals were quickly rejected. In today’s environment on Capitol Hill, NASA is lucky to still be around.

Technically speaking, we could spend the next century studying Mars and find something brand new and scientifically exciting every time. We do that on Earth all the time and we study it every day. But there’s an entire solar system beyond Mars with equally significant scientific wonders to discover and equally compelling reasons to study. NASA doesn’t exist to repeat its last success; its job is to boldly go new places and undertake ambitious missions with uncertain results. It has to stop marketing itself as the agency that once took humans to the Moon and start carving out an identity as a proving ground for high risk but very high payoff blue sky ideas, like DARPA. Will it be an uphill fight to get the attention and funding from politicians whose primary preoccupation today tends to be losing maturity contests to middle schoolers, and a public which likes to keep demanding progress and innovation without caring how its obtained or how much it costs? Yes, it will. But it’s a fight worth having and avoiding it by launching rovers to Mars only delays it…

As if a planet as massive as Saturn being hurled around its star in an orbit that takes less to complete than a typical work week here on Earth, and tidally locked as the kind of heat that can melt rock and vaporize metals bakes its day side wasn’t weird enough, how about taking the same planet and putting its hottest place away to the side, about 80° off where it should be? Sounds odd, doesn’t it? If the planet’s hemisphere was always baked by a star, a hot spot should be front and center at the equator, with slight offsets to account for the wind and endless storms of a gas giant. And yet, the planet Upsilon Andromedae b has its hot spot off to the side, next to the transition between the day hemisphere and the night hemisphere. Why? The highly technical and peer reviewed explanation of this finding can best be summarized by a shrug because we really don’t know.

Winds seem to be the most probable culprit here because most ideas tend to rely on some hidden motion of superheated and chilled gases in Upsilon Andromedae b’s atmosphere. Since the planet doesn’t spin on its axis, or at least shouldn’t due to the tidal pull of the Sun-like yellow-white star it orbits, rotation whipping huge, superheated storms towards the night side wouldn’t really work as a viable explanation. If it was the motion of storms in general, radiating from the center of the planet’s day side and colliding with the colder clouds at the terminator (the twilight areas between the night and day hemispheres of tidally locked worlds), we would see a band of heat around Upsilon Andromedae b’s vertical midsection. Another option could be a heat sink that’s formed by the planet’s turbulent storms. But if there’s a cyclone of cool air attracting the heat being generated at and around the equator and perpetuated by constant sunlight, why is it at an 80° offset, and why is it only in one big blotch on the planet’s side? Why aren’t there multiple heat sinks being formed?

Now remember, one might think that tidally locked gas giants have a blisteringly hot day hemisphere while on the other side of the planet, the night hemisphere is in a perpetual icy chill. But a number of new models over the last few years seem to indicate that the dynamics of Hot Jupiters are more complex than that. Winds from the day side actually warm the night side and the places were the superheated air collides with cooler clouds at the terminator aren’t necessarily home to hyper-violent cyclones. Those effects should only manifest on an eccentric gas giant that quickly swings by its star and gets a sudden influx of energy in the process. So one of the factors to consider in this bizarre finding is that the atmospheres of gas giant close to their stars should be calmer than we think and do a better job of convecting the heat with which they’re bombarded. Though if a subsequent observation of Upsilon Andromedae b confirms that its anomalous hot spot isn’t a mistake or an illusion of some sort, we may have to revise those predictions of relatively more docile Hot Jupiters…

Oh those scientists with their constant corrections. Slightly more than a century ago, they said our planet and the entire solar system was a few hundred million years old, then they said it was 4.56 billion years old after fiddling around with radioactive isotopes in asteroids and meteors. Now, they’re changing the age of the solar system once again. How can we trust them after decades of jumping around and constant re-measuring? So how old is our solar system supposed to be now? About 4.5682 billion years? But wait, that’s a correction of only 300,000 to 2 million years from the date we have now. What’s the big deal? Well, the big deal is that this date solves a small controversy in planetary science and allows scientists to even further refine how to better measure the ages of astronomical objects based on radioactive decay, and how to prepare their samples.

bombardment

While we can say with great certainty that our solar system is between 4.56 and 4.57 billion years old, getting to a more accurate number was a little tricky. Ordinarily, chemists measure the amounts of isotopes formed from the decay of more unstable elements. Since this decay happens at a very steady rate, you can compare the relative amounts of isotopes in a sample of a meteorite and come up with an accurate age. But there was a bit of a snag with dating the isotopes of lead, aluminum and magnesium. The latter two seem to be several million years older than the lead, which is a little odd to say the least. So a team of geochemists decided on a serious look into those troublesome lead isotopes, particularly 206Pb and 207Pb, formed by the decay of two isotopes of uranium. And they weren’t just going to take another measurement. They used another meteorite and washed it with a cocktail of acids to remove every last bit of contamination they could before testing.

Well, wouldn’t you know it, the results from the lead isotopes now match up with the older dates, showing that the readings of aluminum-26 and magnesium-26, were right, and that the other lead isotopes were probably contaminated with something that slightly offset their ages during measurements. Problem solved. But wait, you may ask, will we have another correction to the age of the solar system in the future? After all, if we’re now discussing one, maybe another technique will yield another estimate? And it very well could. However, I would hesitate to label this 300,000 to 1.9 million year refinement to a 4.568 billion year old system a correction and posit that it was in fact just a refinement necessary to understand how the infant solar system formed, but not very meaningful to those of us who are little more than informed laypersons on the subject. For us, the age of the planet or the solar system hasn’t actually changed at all in absolute terms, and we can just as confidently say that Earth is slightly over 4.5 billion years old. And continue to make fun of those who disagree, especially if they try to use the headlines about this refinement as evidence that “scientists keep changing their story.”

See: Bouvier, A., et al. (2010). The age of the Solar System redefined by the oldest Pb–Pb age of a meteoritic inclusion Nature Geoscience DOI: 10.1038/ngeo941

[ illustration by Mario Iliev ]