The "Phantom" Planet

There have been quite a few news articles this past week on the discovery of a "phantom" planet or "invisible" planet. Before I start sounding negative, this is a big find! Okay, now for the slightly negative: this planet that has been found is not that magical. The only unique thing about it is that its too small for us to detect using any other means of detection. But it does exist, and it is cool, mostly because of the way it was found.

Most of the time when planets are detected we never actually see the planet. There are a couple rare exceptions, mostly in very young stellar systems when the planets are still really hot and haven't cooled off at all (our solar system left this phase billions of years ago, so its a short-lived period of time). But what about all the planets being found? They're found using one of two methods: the transit method and the radial velocity method. The transit method is the more conceptually simple of the two. Let's say you were an alien looking at our solar system and just staring at our sun. And lets say you were looking at 'edge-on,' so to speak, so that you could see all the planets pass in front of the sun. Well, when a planet passes in front of a star, it will block out some of the light, similar to an eclipse. Planets don't tend to be very bright compared to the sun, so you, as an observer far away, would see the sun get slightly dimmer. If you saw this dimming happen in a predictable pattern (for example, the Earth would pass in front of the sun every year) you could say that there must be some planet there. With a few more details (like how much the sun got dimmer, how long the dimming lasted) you can find out a few things about the planet that let you get headlines like "Earth-size planet found around nearby star" or something.

The other method is the radial velocity method. You know how when a fire truck moves by you the sound of the siren changes? Its because of something called the Doppler Effect. As an object moves towards you the sound has a higher pitch, or frequency, than normal. When the object moves away from you, the opposite happens. The pitch is lower. The same thing happens with light. So if a celestial object (like a star) is moving towards us, the light increases in frequency, while if it moves away from us, the frequency decreses.

We can measure this increase or decrease in light even if stars are light years away from us because they're so bright, and because telescope are very, very large. So what makes the star move? Planets. We all know that planets orbit around the sun, but that's not entirely true. The Earth goes around the sun, but the sun moves too. It moves a lot less because the sun is a lot bigger than the Earth but it responds to the Earth's motion nonetheless. We can observe stars light years away moving in response to planets orbiting them (and the stars responding), which is how we find about half of the planets we know about.

Okay, now what about these 'phantom' planets. This goes back to the first method of planet detection, known as the transit method. If there's just one planet going around another star, then that planet will orbit in a very predictable pattern. After you see the star get dimmer a couple times, you can measure the length of the dimming and the time in between the dimming and you can actually predict when the next transit event (planet passing in front of the star) will happen to the exact minute, sometimes even better. But what if there's another planet in the system? What happens then? Then your predictions will be off by a little bit, maybe a couple minutes here or there. So if you look for variations in the time between transits (aka transit timing variations) you can say something about whether or not there is a planet or a moon or something else in that system. You can constrain what it could be using simulations but the general idea is that you can use this to find planets.

So why are the planets 'phantom' planets? Usually they're planets we wouldn't be able to detect using the transit or radial velocity methods. This means that they tend to be smaller planets. Which is great for finding things of astrobiological interest, like Earth-like planets. The planet that was found recently is probably too big to be Earth-like, but astronomers are tracking all sorts of transiting systems looking for these 'phantoms'.

The Oldest Evidence of Life

Scientists from Australia and the UK have made a stunning claim - that they have discovered a fossil of microbial life in 3.4 billion year-old rocks. Claims like this have been made before for even older life, but if correct, this result would provide the most concrete microfossil evidence for life in this age range. This would provide a complement to other evidences of life, including stromatolites and isotopic evidence, which is less direct than microfossils.

Sulfur isotopes show evidence for life going back to nearly 3.5 billion years ago. Sulfur isotopic data from Warrawonna in Australia show a strong signature of isotope fractionation. This means that they detected a characteristic signature of life from comparing the amounts of different sulfur isotopes. However, finding evidence of microfossils (fossils of microbial life) has proved to be more elusive. Microfossil evidence is wrought with complications. Essentially with microfossils you have to be able to rule out any possibility except biology. And even then, there's always the possibility that the signal is contamination; that it is a fossil of more recent life that contaminated the old rock.

I'm not a true expert in this, only an academic tourist, but the idea that life existed 3.4 billion years ago is consistent with other data, including isotopic data and stromatolites.

Viruses in Life

Are viruses life? Probably not - even though they have genetic code and protein they can't reproduce without invading a living cell. But they can have an immense impact on how life evolves.

Most of the time when we hear about a virus in the news, its something bad. Viruses cause all sorts of diseases, everything from the flu to HIV. However, viruses might actually have been the most important factor in the evolution of complex life. Here's how:

Most of the time when people think about evolution they think about mutations. A mutations is basically a random error in the reproduction of an organism's genetic code. Sometimes these random errors are bad, most of the time they don't do much of anything, and every once in a while they can actually be good for the organism in whatever environment its in. But mutations alone are not adequate to explain for complexity. Instead something called lateral gene transfer is necessary.

Lateral gene transfer is the transfer of genetic code from one organism to another such that the new code becomes a part of the DNA or RNA of the organism. Lateral gene transfer is in like the internet of evolution. Information is available, and an organism can make the available genetic code part of its own code, just like you can find pretty much anything you need on the internet (or course, microbes can't search for what they want, but I'm tryin' here). Mutation is more like trial and error by an individual trying to figure something out. Think 13th century alchemist trying to come up with a new material by experimenting day after day. Both are useful, but lateral gene transfer was probably much more productive in evolving life early on. This can be done in a few ways, including contact between cells, incorporation from the environment, and also by viral infection.

So, how important is this anyways? Consider a few facts. First, viruses are the most abundant 'biological' entity on the Earth. Also, lateral gene transfer may be the most important mechanism for acquiring new genes. Finally, viruses are known to have useful genetic code; some marine phages (viruses that attack marine bacteria) contain the code necessary for photosynthesis. These facts put together imply that viruses may have been responsible for spreading many genetic innovations in early life. One scenario could be that one organism developed the ability to use photosynthesis for light. Somehow, a virus gets that genetic code necessary for photosynthesis. The virus multiplies, 'infecting' other organisms with the photosynthesis genes. Soon, photosynthesis becomes ubiquitous, or common, throughout the environment.

Viruses are important. While they might seem bad at present day, in the long-term evolution of life on this planet, viruses may have been essential through lateral gene transfer. In fact, without viruses, it is unlikely that complex organisms like human beings could exist.

Sources: Planets and Life, Sullivan & Baross

The Origin of Life

Two recent scientific articles could have an impact on the origin of life on Earth and elsewhere in the universe. The first comes from via Science Daily, concerning the finding of DNA building blocks in meteorites. This is not exactly new - scientists have been finding components of DNA in meteorites for decades. The significance is that there was always a question of whether the components where truly made in space or if they were contamination from Earth organisms. Amino acids, which are the building blocks for proteins, have also been detected in meteorites. The story told by these discoveries is quite compelling - that these meteorites came from asteroids that are essentially acting as 'factories' for organic molecules like amino acids and DNA bases.

The team of NASA scientists were able to validate the DNA bases as non-terrestrial using three tests. The first was that they found molecules that are analogues of DNA bases in the meteorites. Life on Earth is in fact very selective - we as a planet only use 20 amino acids and 5 nucleobases (such as the 4 that are used as DNA bases). However, there are many more possible molecules that life could use. We're still not sure why life has chosen the molecules it has. The scientists were able to find molecules that are not used by life. If the organic molecules were from terrestrial contamination from life on Earth, you would not expect to see the organic molecules that life does not use.

The second piece of evidence is that the concentration of the DNA bases in the meteorites is much greater than the concentration of the bases in the areas around where the meteorites were found (typically Antarctica). This means that for this to be contamination from Earth life, the molecules would have to find some way to purposefully gather to the meteorite as opposed to spreading out throughout the area evenly. The last piece of evidence, which is more indirect than direct, is that it is possible to construct these molecules with biologically. In fact, a plausible set of reactions has been developed and studied in the lab.

This is a tremendously important astrobiological discovery, if correct. We know that asteroids and comets hit the Earth today, and probably did so with greater frequency early in the Earth's history. If those asteroids and comets were bringing the building blocks of proteins and DNA to the surface of the Earth, the chance of life developing increases. It should be noted that this is different from the idea of panspermia, which is much more speculative. The theory is that life on Earth was seeded by asteroids or comets which carried life from elsewhere, potentially even another solar system.

One of the great things about astrobiology is that it truly involves everthing from astronomy to biology, often with drastically different fields approaching the same problem from different angles. For example, the second bit of science news for the day was that scientists working at the Scripps Research Institute have come one step closer to solving a mystery regarding life on Earth. As previously mentioned, Earth life uses a very select sample of potential DNA bases and amino acids for everything. Additionally, amino acids come in two different types, left-handed and right-handed. The difference is more or less superficial - both types can perform the same function. They could both be used in life, but for reasons that are still largely a mystery, life on Earth uses left-handed amino acids. The research of Jason Hein, Eric Tse and Donna Blackmond provides some clues to why this is the case.

The experiment produced a resulting mixture of molecules with a single chirality from an starting solution that was nearly equal in left- and right-handedness. This has been one of the big problems in the process of trying to synthesize RNA. (Note that this is way outside of my area of expertise, so I can provide only a surface treatment of this). If they have come closer to solving the chirality problem it would be a great step forward for the field on the origin of life.

The Earth with Two Moons

Could the Earth have had two moons in the past? This is the theory put forth by UCSC scientists Martin Jutzi and Erik Asphaug. The currently supported theory on the formation of the moon is that early in the history of the solar system (about 4.53 billion years ago) the Earth was impacted by an object roughly the size of Mars. The impact completely melted both of the worlds, and part of the Mars-size impactor became part of the Earth, and the rest was put into orbit around the Earth. Over time, the material in orbit around the Earth came together and formed our Moon. This theory explains many unusual features about the moon, such as its relatively large size and that its isotopic compositions are identical to the Earth's, and different from everything else in the solar system. However, there are still some mysteries about the moon. One problem that is addressed in this theory is that the moon has two very different hemispheres. The nearside of the moon is dominated by cooled lava flows and the farside of the moon is much more mountainous and cratered.

The proposed solution to this dichotomy between the near and far sides of the moon is that during the explosive moon-forming impact, two moons formed instead of one. Under most circumstances, this is highly unlikely. However, if the second moon were to form at a point of orbital stability with the Earth and the other moon, called a Trojan point, it would be possible to have a stable configuration with two moons for tens of millions of years. Eventually the second moon's orbit would be disturbed, sending it on a collision course with the larger moon.

Jutzi and Asphaug simulated this type of collision and varied the size, composition and velocities of the two different moons. The larger of the two moons would still have a magma ocean based off of thermal evolution models. While their simulations cannot predict specific features of the lunar surface, they can predict the overall composition of the surface and look at the near and far side discrepancy.

The primary findings are that a smaller moon moving at speeds lower than sound speed in silicates will not produce a crate, but will eventually re-create the lunar surface we see today. The smaller moon ends up displacing the magma ocean from one side of the moon and pushes it to the other side. The magma ocean becomes the near side of the moon we have today, and the smaller moon gets spread out over the other hemisphere, which becomes the far side of the moon we have today.

There are other plausible ways to create the surface features we see on the moon, so this model can be viewed as one more option in a the pile of possibilities. Thankfully there are experiments that can be done to either confirm or refute this theory. Going to the moon and looking for some sort of compositional difference that could arise from having two initial moons is a possibility, but the differences would likely be small considering that the two moons would have been made of the same material. Another intriguing possibility is using lunar seismology to get more detailed measurements of the crust, or using gravitational measurements. If materials were concentrated only on one half of the moon, this should be detectable by future work by NASA.

Flowing Water on Mars?

A group of scientists using data from the Mars Reconnaissance Orbiter (MRO) has detected possible evidence of flowing water on the surface of Mars. A team led by Alfred McEwen has published this tantalizing result in the journal Science. They have found evidence of seasonal variations on steep slopes in the southern hemisphere of Mars that may be indicative of flowing water.

There is considerable evidence that Mars had a wet past, but most scientists would consider Mars a dry and (most likely) dead world today, at least on the surface. This discovery shows possible evidence for surface water today, and is we have seen on Earth, where there is water, there is life. Images from MRO show dark markings ranging in width from 0.5-5 meters (2-16 feet) on steep slopes that change seasonally. They call these markings recurring slope lineae (RSL). From late spring to early fall they observed the RSL appearing and growing in size; from fall through winter the RSL decrease in size and faded away. The theory is that as the surface heats up during the warmer seasons, water is more likely to be in liquid form on the surface. During the winter, the temperature is too cool for water to exist in liquid form, so it either freezes or sublimates (turns into gas).

The water flowing on Mars isn't exactly like a river or other bodies of moving water on the Earth. For one, this is not pure H2O - this water would be saturated with salts like calcium, magnesium, sodium or maybe iron sulfates.

It should be noted that there are still lots of unanswered questions about the RSL - why are these flows dark? Where is this water coming from? There are theories for the origins of the water, including absorption of water from the atmosphere by salts on the surface or subsurface water seeping out.

The existence of the these markings does not necessarily prove that there is flowing water on the surface of Mars. But the idea of water flowing on the surface of a planet other than Earth is intriguing, and could be astrobiologically significant.