My mental map of the universe has evolved over my life, partly due to new scientific discoveries and partly due to my own education. As if often the case, as you learn more, things get more complicated. The simplistic picture I had as a child was that the universe consisted of many galaxies in which there are many stars and around which there are planets, likely something similar to our own solar system. I have had to modify this model dozens of times, and perhaps I need to make another little tweak. This has to do with where planets exist.

First, galaxies are not randomly distributed throughout the universe. They are bound together in local groups, those groups are gravitationally bound into galaxy clusters, which in turn are part of superclusters which are finally organized into giant filaments, the largest gravitationally bound structures in the universe. So our universal address is – the Sol system within the Milky Galaxy, part of the Local Group within the Virgo Cluster which is part of the Laniakea Supercluster.

At some point I also learned that not all stars (and therefore, not all planets) exist within galaxies. Estimates of the number of stars within and between galaxies just overlap, so they may be equal, but the average estimates indicate that likely 1-10% of all stars are not in galaxies. They are wandering between galaxies, mostly within galaxy clusters. The first intergalactic star was discovered in 1997. It is likely that most such stars were formed within galaxies (you need clouds of gas and gravitational disturbances for stars to form) but then were flung out because of gravitational interactions with other objects, such as a black hole. Two galaxies colliding can also spray their stars throughout the cluster. It is also very likely that such stars would retain their planets.

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Last year the inner solar system had an interstellar visitor – 3I/Atlas (which stands for the third interstellar object which was discovered by the Atlas telescope). The third ever of anything is by definition a rare event, and so this was scientifically exciting. The comet came into the inner solar system, passing close to Jupiter and Mars, but not to the Earth, went behind the sun, then emerged on its path away from the sun. It is now headed for the orbit of Jupiter and out of the solar system. At first 3I/Atlas displayed a number of minor anomalies. It was behaving sort of like a comet, but with some differences. This fits well, however, with the main hypothesis that it is an interstellar comet – so it’s a comet, but may have a different composition from comets that were formed in our own solar system. This is not almost certainly the case – the comet comes from the thick disc of the galaxy, likely from a low metallicity star system, and has likely been travelling through interstellar space for billions of years, possibly being even older than our own star.

Now that it is passing out of the solar system we can look at all the data that NASA collected and make some fairly confident conclusions. There are a lot of sources of information, but Wikipedia actually has a pretty good summary and list of references. In the end, 3I/Atlas behaved mostly like a typical comet. It formed a tail heading away from the sun, brightened as it got close, then faded away as it moved away from the sun. Spectral analysis found that the comet was unusually rich in carbon dioxide (CO2), with small amounts of water ice, water vapor, carbon monoxide (CO), and carbonyl sulfide (OCS). It also had small amounts of cyanide and nickel gas, which is common in comets from our own solar system. In other words – it is a comet. It did originate from a part of the sky that we had previously calculated would have fewer such interstellar objects, which either makes it especially rare or means that our calculations are off.

Every time we encounter a new interstellar object we gather more data about such objects – how frequent are they, where do they come from, and what is their nature. Right now we have just three data points. After the first one, Oumuamua, we had not idea how common they were because we had just one data point. Now we have enough instruments surveying the sky that we are better able to detect such objects, which are very fleeting. The question was – was Oumuamua a one-off, and we just got lucky to detect something that happens very rarely, or are such objects common. We now have three data points and can conclude that they are fairly common, and we should detect one every few years or so, perhaps even more often if we start looking more.

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How common is life in the universe? This is one of the greatest scientific questions, with incredible implications, but we lack sufficient information to answer it. The main problem is the “N of 1” problem – we only have one example of life in all the universe. So we are left to speculate, which is still very useful when based on solid scientific evidence and reasoning. It helps guide our search for signs of life that arose independently from life on Earth.

One important question, therefore, is where is it possible for life to exist? We know life can arise on a rocky planet with a nitrogen and CO2 atmosphere in a temperature range that allows liquid water on the surface. We also know that such life may create and sustain large amounts of oxygen in the atmosphere. It therefore makes sense to focus our search on similar planets. But life does not have to be restricted to Earth-like life. Scientists, therefore, try to imagine what other conditions might also support some kind of life. It is possible, for example, that life arose in the vast oceans under the ice of moons like Europa or Enceladus. Such life would be very different than most life on Earth. It would be dependent on chemical processes for energy (chemosynthetic), rather than sunlight.

Knowing how many different kinds of places life could possibly exist affects our estimate of the number of locations in our galaxy that might harbor life. The current estimates for how many Earth-like exoplanets there are in the Milky Way galaxy ranges from 300 million to 40 billion, depending on various assumptions and how tightly you define “Earth-like”. There are 100-400 billion stars in the galaxy, but about a third of those stars are in multi-star systems, so that means there are tens to up to 100 billion distinct stellar systems in the Milky Way.  One estimate from observed multi-star systems is that about 89% of them could allow for a stable orbit of a rocky planet in the habitable zone.

But perhaps we should not limit the calculations of how many worlds in the galaxy may support life to Earth-like planets. I am not just talking about life in oceans under icy moons. Astronomers have also been considering the possibility of life on moons that orbit free floating gas giant planets. A free floating planet (FFP), also called a nomadic planet or rogue planet, does not orbit a star at all. At some point, likely early in the life of its parent star, it was flung out of its system and now wanders freely between the stars. Astronomers estimate there may be hundreds of billions of such planets in the Milky Way. But this means the planet is dark, without any sunlight to keep it warm or fuel life. What about the moons of an FFP, however?

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If we are going to have an enduring presence on either the Moon or Mars, or anyplace off of Earth, we will need to grow food there. It is simply too expensive, inconvenient, and fragile to be dependent on food entirely from Earth. In fact, any off-Earth habitat will need to be able to recycle most if not all of its resources. You basically need a reliable source of energy, sufficient food, water, and oxygen (consumables) to sustain all inhabitants, and the ability to endlessly recycle that food, water, and oxygen.

The ISS has achieved 98% recycling of water, which is what NASA claims is the threshold for sustainability of long space missions. The ISS also recycles about 40% of its oxygen. However, the ISS grows none of its food. It is all delivered from Earth, with a 6 month supply aboard the ISS. There are experiments to grow plants on the ISS, and these have been successful, but this is not a significant source of nutrition for the astronauts.

Doing the same on the Moon is not practical for long missions, although we will certainly be doing this for a time. But the goal, if we are to have a lunar base as NASA hopes (NASA plans a lunar base at the Moon’s south pole by 2030) is to grow food on the Moon (and eventually on Mars). On the ISS the big limiting factor is microgravity. The Moon has lower gravity than Earth, but it has some gravity and so that will likely not be a major problem, especially since we can grow plants on the ISS. We can also grow plants hydroponically pretty much anywhere, and I suspect this will happen on any lunar base. But a fully hydroponic system has its limits as well.

Hydroponics on the Moon would be challenging for several reasons. First, it is energy intensive, and energy may be a premium on a lunar base, especially early on. Second, it requires a precise balance of nutrients in the water, and those nutrients would have to be sourced from Earth. So it doesn’t really solve the problem of dependence on Earth. And third, hydroponics requires a lot of equipment which would have to be shipped from Earth. We could theoretically leach nutrients from lunar regolith, and this might help a bit, but is also energy intensive and would not be a source of nitrogen.

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As we continue the search for life outside of the Earth, it helps if we have a clear picture of where life might be. This is all a probability game, but that’s the point – to maximize the chance of finding the biosignatures of life. One limitation of this search, however, is that we have only one example of life and a living ecosystem – Earth. Life may take many different forms and therefore exist in what we would consider exotic environments.

That aside, it seems a good bet that life is more likely in locations where liquid water is possible, and therefore liquid water is a reasonable marker for habitability. When we talk about the habitable zone of stars, that is what we are talking about – the distance from the star where it is possible for liquid water to exist on the surface of planets. There are more variables than just the temperature of the star, however. The composition of the atmosphere also matters. High concentrations of CO2, for example, extend the habitable zone outward. There is therefore a conservative habitable zone, and then a more generous one allowing for compensating factors.

A new paper wishes to extend the conservative habitable zone further, specifically around M and K class dwarfs. K-dwarfs, or orange stars, are likely already the best candidates for life. They are bright and hot enough to support liquid water and photosynthesis, they emit less harmful radiation than red (M) dwarfs, and live a relatively long time, 15-70 billion years. They also comprise about 12% of all main sequence stars. Yellow stars like our sun are also good for life, but have a shorter lifespan (10 billion years) and make up only about 6% of main sequence stars.

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South Korean astronomers are challenging the notion that the universe’s expansion is accelerating, an observation in the 1990s that lead to the theory of dark energy. This is currently very controversial, and may simply fizzle away or change our understanding of the fate of the universe.

In the 1990s astronomers used data from Type Ia supernovae to determine the rate of the expansion of the universe. Type Ias are known as standard candles because they put out the exact same amount of light. The reason for this is the way they form. They are caused by white dwarfs in a double star system – the white dwarfs might pull gas from their partner, and when that gas reaches a critical amount its gravity is sufficient to cause the white dwarf to explode. Because the explosions occur at the same mass, the size of the explosion, and therefore its absolute brightness, is the same. If we know the absolute brightness of an object, and we can measure its apparent brightness, then we can calculate its exact distance.

The astronomers used data from many Type Ia supernova to essentially map the expansion of the universe over time. Remember – when we look out into space we are also looking back in time. They found that the farther away galaxies were the slower they were moving away from each other, as if the universal expansion itself were accelerating over time. This discovery won them the Nobel Prize. The problem was, we did not know what force would cause such an expansion, so astronomers hypothesized the existence of dark energy, as a placeholder for the force that is pushing galaxies away from each other. This dark energy force would have to be significant, stronger than the gravitational force pulling galaxies together.

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The universe is a big place, and it is full of mysteries. Really bright objects, that can be seen from millions or even billions of light years away, can therefore be found, even if they are extremely rare. This is true of fast radio bursts (FRBs), which are extremely bright and very brief flashes of light in the radio frequency. They typically last about one thousandth of a second (one millisecond). Even though this is very brief, they still represent a massive energy output, and their origins have yet to be confirmed.

Recently astronomers have detected the brightest FRB so far seen, and it was relatively close, only 130 million light years away. That may seem far, but most FRBs are billions of light years away (again, indicating that they are relatively rare, because we need a huge volume of space to see them). Because this FRB was bright and close, it gives us an opportunity to examine it in more detail than most. But – this is also made possible by recent upgrades to the equipment we use to detect FRBs.

The primary instrument we use is CHIME (Canadian Hydrogen Intensity Mapping Experiment). As the name implies, this was developed to map hydrogen in the universe, but it is also well-suited to detect FRBs. So far, since 2018, it has detected about 4,000 FRBs. But because they are so brief, it is difficult to localize them precisely. We can see what direction they are coming from, and if that intersects with a galaxy we can say it probably came from that galaxy. But astronomers want to know where within that galaxy the FRB is coming from, because that may provide clues to confirm their origin. So they built “outriggers” – small versions of CHIME spread around North America to effectively increase the size of the CHIME detection area and significantly increase its precision. It was this new setup that detected the recent FRB. What did they find?

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Avi Loeb is at it again. He is the Harvard astrophysicist who first gained notoriety when he hypothesized that Oumuamua, the first detected interstellar object, might be an alien artifact. His arguments were pretty thin, not taken very seriously by the scientific community, and mostly did not pan out. However, Oumuamua has left the solar system and so any unanswered questions will remain forever unanswered. But Loeb has been riding his fame and his alien artifact narrative ever since, founding the Galileo Project dedicated to looking for alien technological artifacts. Recently, NASA discovered the third every interstellar object, and the first interstellar comet, 3I/ATLAS. (“3I” is simply the nomenclature for the third interstellar object, and “ATLAS” for the Asteroid Terrestrial-impact Last Alert System that made the discovery.) Loeb recently published a paper and is blogging that this too shows “anomalies” deserving of exploring the ET technology hypothesis. And again – I am not impressed.

Let me jump ahead a bit and say up front – I am not against exploring the alien hypothesis, pretty much in any context. Even though the probability may be low, the payoff would be huge, and it’s worth a consideration. I am not against looking for alien technological signatures. This may, in fact, be the best method for detecting an alien technological civilization. I also think that serious academics and scientists should be taking such efforts seriously and there should be no academic shame in engaging in them. So I am with Loeb to that extent.

What bothers me about Loeb is that his arguments are so terrible. He is just another classic example of an academic and scientist who has no apparent experience with scientific skepticism and therefore is falling for common pitfalls. He also appears to not have learned anything in the last seven years, which is greatly disappointing. In fact, I would argue that he is hurting his stated greater cause (with which I largely agree), to make searching for alien technology academically respectable. Loeb is essentially engaging in anomaly hunting, and shows no signs of understanding what that means. Let’s take a look at his latest list of apparent “anomalies” to see what I mean.

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Let’s talk about climate change and life on Earth. Not anthropogenic climate change – but long term natural changes in the Earth’s environment due to stellar evolution. Eventually, as our sun burns through its fuel, it will go through changes. It will begin to grow, becoming a red giant that will engulf and incinerate the Earth. But long before Earth is a cinder, it will become uninhabitable, a dry hot wasteland. When and how will this happen, and is there anything we or future occupants of Earth can do about it?

Our sun is a main sequence yellow star. The “main sequence” refers to the Hertzsprung-Russell diagram (HR diagram), which maps all stars based on mass, luminosity, temperature, and color. Most stars fall within a band called the main sequence, which is where stars will fall when they are burning hydrogen into helium as their source of energy. More massive stars are brighter and have a color more towards the blue end of the spectrum. They also have a shorter lifespan, because they burn through their fuel faster than lighter stars. Blue stars can burn through their fuel in mere millions of years. Yellow stars, like our own, can last 10 billion years, while red dwarfs can last for hundreds of billions of year or longer.

Which stars are the best for life? We categorize main sequence stars as blue, white, yellow, orange, and red (this is a continuum, but that is how we humans categorize the colors we see). Interestingly, there are no green stars, which has more to do with human color perception than anything else. Stars at an otherwise “green” temperature have enough blue and red mixed in to appear white to our color perception. The hotter the star the farther away a planet would have to be to be in its habitable zone, and that zone can be quite wide. But hotter stars are short-lived. Colder stars last for a long time but have a small and close-in habitable zone, so close they may be tidally locked to their star. Red dwarfs are also relatively unstable and put out a lot of solar wind which is unfriendly to atmospheres.

So the ideal color for a star, if you want to evolve some life, is probably in the middle – yellow, right where we are. However, some astronomers argue that the optimal temperature may be orange, which can last for 15-45 or more billion years, but with a comfortably distant habitable zone. If we are looking for life in our galaxy than orange stars are probably the way to go.

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Exoplanets are pretty exciting – in the last few decades we have gone from knowing absolutely nothing about planets beyond our solar system to having a catalogue of over 5,000 confirmed exoplanets. That’s still a small sample considering there are likely between 100 billion and 1 trillion planets in the Milky Way. It is also not a random sample, but is biased by our detection methods, which favor larger planets closer to their parent stars. Still, some patterns are starting to emerge. One frustrating pattern is the lack of any worlds that are close duplicates of Earth – an Earth mass exoplanet in the habitable zone of a yellow star (I’d even take an orange star).

Life, however, does not require an Earth-like planet. Anything in the habitable zone, defined as potentially having a temperature allowing for liquid water on its surface, will do. The habitable zone also depends on variables such as the atmosphere of the planet. Mars could be warm if it had a thicker atmosphere, and Venus could be habitable if it had less of one. Cataloguing exoplanets gives us the ability to address a burning scientific question – how common is life in the universe? We have yet to add any data points of clear examples of life beyond Earth. So far we have one example of life in the universe, which means we can’t calculate how common it is (except maybe setting some statistical upper limits).

Finding that a planet is habitable and therefore could potentially support life is not enough. We need evidence that there is actually life there. For this the hunt for exoplanets includes looking for potential biosignatures – signs of life. We may have just found the first biosignatures on an exoplanet. This is not 100%. We need more data. But it is pretty intriguing.

The planet is K2-18b, a sub-Neptune orbiting a red dwarf 120 light years from Earth. In terms of exoplanet size, we have terrestrial planets like Earth and the rocky inner planets of our solar system. Then there are super-Earths, larger than Earth up to about 2 earth masses, still likely rocky worlds. Sub Neptunes are larger still, but still smaller than Neptune. They likely have rocky surfaces and thick atmospheres. K2-18b has a radius 2.6 times that of Earth, with a mass 8.6 times that of Earth. The surface gravity is estimated at 12.43 m/s^2 (compared to 9.8 on Earth). We could theoretically land a rocket and take off again from its surface.

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