Where is Everyone?
While Earth is the only place we know that supports life, the universe is a big place. So why not look around? But where would we find life? There are a couple good candidates for extraterrestrial bodies that may contain life. Some of them are in our neighborhood, some a little further away.
But before we get into that, just what does a planet need to support life? As it turns out, an important thing for life on Earth and probable life elsewhere is water.
Yes, water. Specifically liquid water. Liquid H2O is perhaps the most important chemical involved in life processes. Mainly because it’s a very good solvent. Actually, it dissolves nearly everything. This is a big deal when you need to get nutrients from one place to another. There are other reasons why water is so important. However, since this particular article is about extraterrestrial places that may contain life and not the chemistry of water, here’s a link to an article which further explains the awesomeness of water.
So what does water need to dissolve? CHNOPS, or: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. These are the elements that make up the majority of life here on Earth. So, it stands to reason that life elsewhere would need these elements as well.
It is important to note that there are theories which allow for more exotic biochemistries, such as life based on silicon (which replaces carbon) or ammonia (which replaces liquid water as a solvent). Before we get too bogged down with alien rock monsters, notice that when you look at these elements, they are really just chemical analogues of what makes up life here on Earth. So, theoretically, you just need chemical equivalents to CHNOPS and a decent solvent.
There you have it. The recipe for life:
- One pot of liquid H2O (if necessary, substitute ammonia, methane, or hydrogen fluoride)
- Add CHNOPS (if necessary, substitute silicon for carbon, chlorine for oxygen, arsenic for phosphorus, selenium or tellurium for sulfur)
- Bake on high heat for a few million years
Now that we know what we need for life, where do we look? Well, I’m glad you asked. Let’s start with a few neighbors.
I know most people would start with Mars, but we already know a fair bit about Mars. We’ve sent a good amount of satellites to Mars to take pictures. We’ve sent a fair amount of robots to poke and prod Mars. We’ve got nothing.
Now this is not to say Mars is uninteresting, does not contain life, or did not at one time support life. It is to say I don’t feel like writing about Mars. If you really want to delve into life on Mars here’s a good place to start. Now, let’s talk about Europa.
Europa is Jupiter’s sixth largest moon. The surface of Europa is chilly. Really chilly: -260° F to -370°F chilly. This would typically rule Europa out as a contender for extraterrestrial life. However, the chilly surface of Europa is ice. That’s right, solid water.
Now, solid water is not liquid water, and liquid part is important. So with Europa you need to dig a little deeper. Thirty to 200 km deeper, to be more specific.
How could we get liquid water when the surface is so cold? We would need energy and, due to tidal forces between Europa and Jupiter, we’ve got energy. It is similar to what the moon does to the Earth’s oceans, except greatly exaggerated due to the mass of Jupiter. The extreme effect of Jupiter’s tidal forces can be seen on Io, the most geologically active body in our solar system. These forces provide a heat source which could heat the subsurface of Europa and presto chango, liquid water.
The type of life you might find on Europa would most likely be similar to life found on Earth at hydro-thermal vents found on the ocean floor. While most living beings at or near the surface of the Earth depend on organisms that use photosynthesis to create complex sugars, life at the bottom of the ocean depends on organisms that use chemosynthesis to create complex sugars.
Not exactly E.T., but beggars can’t be choosers. There have already been several probes which have taken pictures and analyzed Europa. There have even been various suggestions of landing types of heat-drill-equipped-robots on the surface to drill through the ice and take a peek at anything Europa is hiding. Unfortunately, it looks as if none of these projects are close to becoming a reality. So until they do, we can only imagine what odd creatures might be below the ice of Europa.
Remember how you don’t necessarily need water and CHNOPS for life? Well here’s a candidate for that theory. Titan is Saturn’s largest moon, and much like Europa it’s cold, -179° F at the surface. However, unlike Europa, Titan has a substantial atmosphere composed mostly of nitrogen with some hydrogen and hydrocarbons like methane. This is good news for fulfilling the C and N requirements of CHNOPS. In fact, besides Earth, Titan is the only body in our solar system known to have a nitrogen rich atmosphere.
The only problem is at – 179° F water pretty much only exists as a solid. But there is methane, and methane could replace water as a solvent for biological processes. Additionally, at those temperatures, methane is a liquid. In fact, the Cassini-Huygens probe collected data which suggests the existence of methane lakes and even methane rain on Titan. The collection of chemicals on Titan is, in fact, very similar to the chemicals suspected to be abundant in the very early years of Earth.
As you might expect with unconventional biochemistry comes unconventional life. Namely methanogens. Methanogens make their living through a process called methanogenesis. Essentially, these organisms inhale hydrogen and exhale methane. This means that if there is a significant amount of life on Titan it would alter the atmosphere such that there are lower levels of hydrogen and higher levels of methane than expected. And, to a certain extent, this is what scientists found. Admittedly, the most probable reasons for this are more likely to be physical or chemical, not biological. But one can hope.
In terms of exploration, we have actually landed on and taken pictures of Titan, which is more than most places in our Solar System can claim. There are also some future missions planned but, much like Europa missions, none of them are close to reality.
This one is a bit out of the neighborhood. Twenty light years out of the neighborhood to be exact. Discovered in 2007 by scientists at the Geneva Observatory, Gliese 581d orbits the red dwarf star Gliese 581. Unlike our previous contenders, it orbits in the habitable zone, the orbital sweet spot not too close and not too far away from a star. The place where liquid water can exist at temperatures and pressures similar to Earth.
Gliese 581d has the distinction of being one of the first Earth-like exoplanets discovered in the habitable zone, albeit at the edge of the zone.
Not much is known about the composition of the planet or its atmosphere. However, there are models which allow for Earth-like conditions.
The exciting thing about Gliese 581d is that it is one of 940 exoplanets that have been confirmed since 1988 and one of 26 possibly habitable planets found since 2007. Another exciting thing about Gliese 581d is that we are looking for and finding planets orbiting stars other than our own. We are looking for and finding planets like Earth. The exciting thing about Gliese 581d is that we are looking.
Are we alone? For the moment, yes. But that might not always be the case. With new discoveries of exoplanets and planets in habitable zones, it is becoming more likely that the answer to that question is no.
Whether that something else is a microbe below the surface of Europa, or some intelligent being asking the same question on Gliese 581d, it will be nice to know we’re not alone.