July 8, 2017 9:38 pm

8 Frequently Asked Space-Related Questions (Part 2 of 2)

Aliens, Pluto, Dark Energy, and Dark Matter Explained

Today’s guest blog post is by StarTalk intern Kirk Long. Kirk is majoring in physics while minoring in mathematics and piano at Idaho State University. He spends his weekends working at the largest public observatory in Idaho, the Bruneau Sand Dunes State Park Observatory, where he gives educational astronomy presentations and operates various large telescopes for the public.

Continuing on from our post last week wherein we covered the lives and deaths of stars, asteroid impacts, and black holes, we’ve answered four more exciting questions below.

5: Do aliens exist?

The math would certainly say so. Imagine that you have a bag filled with a thousand marbles, one of which is blue and 999 of which are red. Pretend the red marbles represent planets devoid of life, while the blue marble represents life. If you were to stick your hand in at random, you would expect to get a red marble, and if you only did this a few times you might get bored and conclude the universe was devoid of life. However, if you dumped the bag of marbles out onto the floor you would clearly see the blue one standing out amongst a sea of red. Now, if you imagine that the universe is infinite (which as far as we know it very well could be) then in our experiment we actually don’t have just one bag of marbles, but infinitely many bags. And although it is true that we would then have an infinite amount of planets devoid of life, we would also have an infinite amount of planets with life, even if the chances of life occurring are small. So why haven’t we found other life yet? Space is, to quote the great Douglas Adams, “vastly, mind-bogglingly big,” and it’s also very old. We’ve only looked at a tiny percentage of planets across the cosmos, and we’ve only been looking for a tiny fraction of the time that the universe has existed. Maybe life has already evolved and died on some of these worlds, or maybe it’s evolving there now and we will have to wait hundreds, thousands, or even millions of years for that new light and data to reach us. We also simply don’t know how common life really is, and what types of life are more likely. Our own planet was host to single-celled organisms relatively early in its history, but it took much longer for multicellular life to evolve. Recent discoveries on Jupiter’s moon Europa and Saturn’s moon Enceladus indicate that they could potentially host simpler life in subsurface oceans similar to what we have found living near deep sea vents on Earth, so it could be that simpler life forms are relatively abundant but sentient life much more rare. Furthermore, most of our search is confined to looking for life that we would obviously recognize (water dependent and carbon based). It could be that life is incredibly common (maybe 5, 10, or even 100 “marbles” out of every thousand might be blue) and we may get our lucky break soon, or it could be that the chances are so small that we may never be able to detect it simply because the universe is so vast in age and in size. All we know for sure is that there is at least some probability of life existing — we are living proof of that — and that this is one of the questions that will continue to drive us as we explore the final frontier.

6: Why isn’t Pluto a planet?

Basically, the International Astronomical Union (which is in charge of such important matters) has three criteria to be considered for planethood, of which Pluto fails two. First, you must be of a spheroid shape held together under your own gravity — Pluto passes this test, but so does our own moon. Second, you must have cleared your orbit of debris — Pluto fails this because it resides inside the Kuiper Belt. Third, you must orbit the sun — Pluto fails this because its largest moon, Charon, has so much mass relative to Pluto that they actually both co-orbit each other around a point somewhere in between them, so Pluto isn’t just beholden to the sun’s gravity. Ever since the IAU “demoted” Pluto in 2006, there have been pushes to restore Pluto’s status, but for Pluto will remain a dwarf planet. So why was Pluto ever a planet? Well, when we first discovered Pluto in 1930 we didn’t know a whole lot about it or its neighbors (the Kuiper belt wasn’t discovered until 1992), and we didn’t know that other things like it existed. Eventually we found several other objects in our solar system that were more similar to Pluto than the 8 planets, and instead of naming all of these things as planets we decided it would be easier to recategorize them all as dwarf planets.

7: What is dark energy?

A lot of people have heard of dark energy, but they are rightly very confused. It’s a concept that’s been around for a while — one of its earliest proponents was Albert Einstein himself. In Einstein’s first drafts of his papers on relativity, he included something called the “cosmological constant” — an invisible expansionary force he needed to make his equations work in a static universe (scientists at the time had not yet discovered that the universe was expanding). A few years later, when Edwin Hubble discovered that the universe was in fact expanding, Einstein removed it from his equations and reportedly called it the biggest blunder of his career. As we have discovered that this expansion rate isn’t constant, we’ve realized Einstein may have in fact been on to something, even when he thought he was so wrong. Over the years this expansionary force has gone through a few different names, but today the consensus among physicists is that we’re calling it dark energy for an obvious reason — it’s energy affecting the system that is the universe, but we can’t see it (thus making it dark). We can observe its effects on our universe, but we have no idea what it is, what causes it, or why it’s distributed across the universe in the way that it is.

8: What is dark matter?

Like dark energy, dark matter is another mysterious force in the universe that we know of only because we can observe its effects. Some of the best evidence for dark matter comes from the rotational rate of galaxies. Basically, many galaxies are spinning faster than they should be able to. Based on our measurements of the black holes at their centers and the rate that galaxies spin, many are spinning so fast that even the immense gravity of a supermassive black hole should not be enough to keep them together. Stars aren’t being flung off into space however, so something must be holding them together, and that cosmic glue scientists have termed dark matter (although Neil has suggested it might be better to call it “dark gravity”). There are a number of exciting experiments going on right now to try to figure out innovative ways to detect dark matter, and understanding it is critical to our understanding of the evolution of galaxies and the universe as a whole. Scientists recently put together one of the first maps of dark matter (seen above) using a technique called gravitational lensing — basically, the path light travels through space is warped by the gravity of things around it, and if we see warping and can explain the other variables away, what we think must be warping it is dark matter. This map covers less than half of a percentage of the night sky, but researchers are hoping to use this technique to create a much larger map. Although both dark matter and dark energy sound a bit mysterious, they’re actually both well documented and observed phenomenon that just haven’t been fully explained yet. As we collect more data and figure out new ways to peer out into the cosmos, these questions will be better answered (and whoever answers them will probably win a Nobel prize).

There’s a great episode from season 4 where Neil and comic co-host Leighann Lord answer cosmic queries on dark matter and dark energy. It’s definitely worth a listen if we haven’t satiated your appetite for learning yet.

Thanks for reading — did we miss anything? Do you have any burning questions we didn’t get to? Sound off in the comments and we may be able to answer them in a future post.