Scientists have published the most extensive map of dark matter in the universe to date, based on a survey of 100 million galaxies. The findings don’t quite match with predictions made by computer models, suggesting that there is some physics at work which scientists do not yet understand. This, of course, is exciting for physicists.

As I discussed previously, we don’t know what dark matter is, but we are pretty confident it’s there. Dark matter does not give off any radiation, but it does have gravity, so we can see its gravitational effects. Based on these observations it seems that 80% of the matter in the universe is dark matter. This is a major area of research, because we do not know what dark matter is made of. It is probably some new particle we have not identified so far. This is where scientists live – on the edge of our current knowledge, peering into the unknown.

Part of that “peering” is gathering lots of data, and that is what the current study does. They used gravitational lensing to map the gravity of the universe, 80% of which is dark matter. Visible galaxies and dark matter cluster together, creating an overall structure to the universe. There are vast black voids with nothing, and there are tendrils of matter with galaxies, gas, and stars. The goal is to map this distribution, to see where all the stuff in the universe is.

They then compared this map to what we would predict based on our current understanding of the laws of physics. They started with a map of where all the matter was 350,000 years after the Big Bang, which was created by examining the cosmic background radiation. Then they model where that matter should have gone over the last 13.8 billion years based upon relativity and other physical laws. The map and the model were off by a few percent. The universe is more evenly distributed than the models predict. This may not sound like a lot, but physicists are used to dealing with high levels of precision. Physical laws tend to be very reliable. This is why we can make calculations and send a probe to Pluto 5 billion km away, and arrive precisely where they predicted. If the New Horizons probe was off course by a few percent, that would have been a disaster, both for the mission and our understanding of the relevant laws of physics.

This is why physicists love discrepancies between predicted and observed phenomena, even tiny ones. It means something is going on we are not aware of. This could be an effect we have not considered, an error in their experimental design or method of observation, or occasionally a tweak to our understanding of the laws of physics. The first two need to be thoroughly ruled out before new physics can be confidently postulated, and it is an increasingly rare event, but that is what physicists live for.

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I’ve been watching For All Mankind – a very interesting series that imagines an alternate history in which the Soviets beat the US to landing on the Moon, triggering an extended space race that puts us decades ahead of where we are now. By the 1980s we had a permanent lunar base and a reusable lunar lander, not to mention spacecraft with nuclear engines. Meanwhile, back in reality, we are approaching 50 years since any human has stepped foot on the moon.

But NASA does plan on returning to the Moon and staying there this time, with their Artemis mission. (In Greek mythology Artemis was the twin sister of Apollo.) Originally they planned to return to the Moon by 2028, then Trump asked them to move up the timeline to 2024. NASA dutifully complied, but this was never realistic and anyone who has been following Artemis knew this was not going to happen. And now NASA is admitting they will not be ready by 2024. But sometime likely in the latter half of this decade we will return to the Moon.

One of the last pieces to put into place is a lunar lander, something to get people from lunar orbit down to the surface of the Moon. NASA has finally awarded the contract to build this lander – to SpaceX.  They are making no secret of the reason. SpaceX gave the lowest bid, by far. This is partly because the entire mission of SpaceX is to make space travel cheaper, mainly by using as many reusable parts as possible. Toward this end they perfected the technology for landing rockets vertically. The videos of Falcon rockets landing after launching satellites is still stunning. SpaceX also achieved a rating for their Dragon Crew capsule to actually carry people into space, and they have delivered astronauts to the ISS. Finally, SpaceX has already been developing their Starship design, which will be the basis of the new lander, which NASA is calling the Human Landing System (HLS).

Interestingly, a recent independent analysis found that the most efficient (only looking at efficiency) landing system using non-reusable parts was the Apollo system – a two-stage approach with a landing module and ascent module. However, if you use a renewable lander, then the one stage approach makes the most sense. That is in keeping with SpaceX’s philosophy, so it’s not surprising that they are taking that approach. I do wonder if they are going to use an actual Starship just outfitted for lunar landing, or are they going to make a new and smaller version? If the former, then it seems a bit odd that the HLS part of the system is a ship capable (theoretically) of doing the entire mission, from Earth surface to Lunar surface. That is Musk’s vision, single stage to destination for maximal reusability.

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In the 1970s astronomer Vera Rubin was observing the Andromeda galaxy and discovered something very curious. Andromeda is a spiral galaxy, like our own, and spins like a pinwheel. The “spinning” is comprised of all the individual stars (and gas and dust, but the stars are what we can see) revolving around all the mass within their orbit. If you run the numbers, as stars get further away from the galactic center they should revolve more slowly. The relationship between distance from the galactic center and each star’s velocity is called a galactic rotation curve. Rubin and others predicted the curve should be decreasing in a linear fashion (after an initial rise because of the increase in mass at the galactic center).

What Rubin found, however, was that the rotation curve of Andromeda increased at first as expected but then did not decrease with distance but remained largely flat. This difference between prediction and observation was a genuine anomaly and required an explanation. The results were verified with other large spiral galaxies, and yes, they all have flat rotation curves. The stars on the outskirts of these galaxies, according to Newton, should be flying away. They are moving to fast to be held by the gravity of the galaxy they are orbiting.

Unless…

Perhaps there is more mass in the galaxy than we can observe. There is matter that is not giving off light like stars or even reflecting or glowing from the light of stars like gas clouds. There must be matter we cannot see, some dark matter. How much dark matter would it take to explain the observed rotation curves? Quite a lot – about six times the mass of the stuff we can see. If true, then some 84% of the matter in the universe is dark matter. And we do not know what dark matter is – we don’t know what most of the universe is made of.

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Right now there are about 3,000 active satellites in Earth orbit. About 1,000 of those satellites are part of the Starlink project to provide internet access everywhere on the planet, with a planned 42,000 total when complete. that is a massive increase in the number of active satellites. At the same time there another 3,000 defunct satellites that are no longer operational but remain in orbit. There is about 9,000 tonnes of total orbital debris, and we are tracking 30,000 objects of 10 cm or larger. But estimates are that there are millions of smaller objects in orbit.

In other words – usable Earth orbit is becoming crowded and hazardous. This is a risk to operational satellites, space stations, and any spacecraft hoping to get off Earth. Much of this debris is moving very fast relative to other objects with intersecting orbits. A lost bolt could destroy a satellite or punch a hole in the International Space Station (ISS). There is a concern that a serious collision, say between two satellites, would generate enough debris to cause a cascading event of further collisions.

There are now international agreements that make states responsible for anything they put into orbit for its lifetime. Companies and nations are supposed to arrange for the deorbiting of anything they put into orbit within 25 years of the end of its functional lifetime. However, the agreements have little teeth and compliance is low. This is just another example of allowing entities to externalize the costs of their own waste or downstream effects. It is also another example of how the assumption that natural resources are so gigantic we don’t have to worry about sustaining them. Space is really big, so who cares if we leave a lot of junk up there? Well, it took only a few decades for us to clutter low Earth orbit with enough debris to be a serious hazard.

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In 2017 astronomers spotted a very unusual object approaching Earth. What was most unusual about it was that it was on a trajectory that would take it out of the solar system. Given its path it could only have come from outside the solar system – our first ever discovered extrasolar visitor, named Oumuamua. For an extrasolar object, it came improbably close to the Earth and the Sun, which gave us a great opportunity to take a close look at it. And then, as it passed by the sun and headed out of the solar system it became even more unusual. First, we could see that it was an very long and flat object, not typical for a comet or asteroid. Second it accelerated as it moved away from the sun, like a comet would from sublimation of ice into gas acting like a rocket. But we could not see a comet-like tail, and the albedo was off. Curiouser and curiouser.

This lead some to speculate wildly that Oumuamua may be an alien artifact, most famously Avi Loeb, a Harvard scientist who has now even published a book – Extraterrestrial: The First Signs of Intelligent Life Beyond Earth. This is a clear case of the “aliens of the gap” fallacy – any astronomical phenomenon we do not currently fully understand must be evidence of alien technology. Of course, all natural explanation must first be excluded. But even then, we don’t have aliens, we have an unknown phenomenon that needs further exploration.

Oumuamua is now yet another great case in point. Two Arizona State University astrophysicists, Steven Desch and Alan Jackson, have come up with a plausible explanation for Oumuamua’s funky properties. Perhaps, they hypothesized, our attempts so far to explain the object’s behavior and properties failed because we were making false assumptions about what kind of ice it might contain. We assumed it would have a profile of ice similar to the comets we know. But what if the ice is made of something else, because Oumuamua is not a typical comet. When they looked at the properties of nitrogen gas – bingo. This would nicely fit the data, including the combination of the rate of acceleration from ice sublimation near the sun and the low albedo – not as much reflective ice would have been necessary to cause the acceleration.

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The discovery and exploration of exoplanets over the last three decades has been an exciting addition to astronomy. In 1990 we knew of no planets outside our solar system, and now there are more than 4,000 confirmed exoplanets, and thousands of more candidates awaiting confirmation. This is still just a tiny sample of the planets even in our small corner of the galaxy. One of the questions going into this enterprise was – how typical are the planets we know in our own system, and also how typical is our system in terms of the number and arrangement of planets? So far the answer seems to be that there is no typical. We are finding all kinds of planets in all kinds of arrangements.

We can now add, potentially, one additional planetary phenomenon to the list – a planet with an apparent secondary atmosphere. The planet is GJ 1132 b and is 41 light years away (in naming convention the star itself has the designation “a” and so “b” is the first planet discovered in the system). This is a red dwarf star, and the planet is very close, so close that it is tidally locked, meaning the same side faces the star at all times. It’s year is only a day and a half. Planets this close to their parent stars will tend to have their atmospheres stripped by the heat and solar wind from the star.

Astronomers believe that GJ 1132 b was once a subNeptune – a planet with a rocky core about the size of Earth, but a thick hydrogen-helium atmosphere making it a gas giant. But soon after formation that atmosphere would have been blown away, leaving behind the rocky core. So astronomers expected to see little to no atmosphere on GJ 1132 b, and instead they find evidence of an atmosphere about as thick as Earth’s. This and other evidence has led them to believe this is a secondary atmosphere.

Stars tend to be hotter when they are very young and then cool down a bit. Red dwarfs change even more. They are unstable when young, giving off lots of coronal mass ejections, constantly blasting any nearby planets. This does not bode well for the prospect of life on any such planets. Unfortunately, any planet in the habitable zone is also in this blast radius and would have its atmosphere stripped. Red dwarfs are the most common star type in the galaxy, making up 70% of the stars, so this has implications for the probability of life in the galaxy.

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Recently experts gathered online for a digital conference in which they discussed possibilities for detecting signs of alien technological civilizations – so called “technosignatures”. Being an enthusiast, I have heard of many of these before, but there were a lot of new ideas coming out of that meeting as well. Here is the preprint, with all the technical information. I think collectively this is a compelling case for NASA and other agencies to include the search for technosignatures as a part of their mission.

The most obvious technosignature is radio signals, most likely deliberately broadcast, either out into the universe or even directed at Earth (if they have detected our own technosignatures). This is the object of SETI (The Search for Extraterrestrial Intelligence). NASA briefly funded a SETI project, but then pulled funding. The effort continues, however, with other funding. I have interviewed Seth Shostak from SETI several times and he makes a couple of points worth emphasizing here. One is that SETI is simultaneously doing a lot of non-SETI astronomy. They are essentially doing radio astronomy but looking at any data in such a way that it could detect an alien signal.

The members of the recent meeting also considered what they call “synergies” or ancillary benefits for each of the techniques they discuss. This is critical, I think, because it means even if we never detect an alien technosignature, the effort would not have been wasted. We will have accomplished a lot of meaningful astronomy in the meantime. I would argue it wouldn’t be wasted in any case – negative results from an experiment are still results. We would have gathered lots of data about how rare technological civilizations are in the universe. But if you look at the table of discussed techniques in the paper, each one has a listed potential synergy. In fact, they also list existing astronomical data that can be searched for technosignatures.

Seth also pointed out that if we did detect an alien message hiding in radio signals, we would very likely not be able to read it. This is because radio signals get weaker with distance, and at some point they would be lost in the background radio noise. The more powerful the initial radio signal, the greater its range. The current authors call this the “cosmic footprint” of each technosignature, and coin the term “ichnoscale” to measure it. For example, they give radio astronomy a 10 kpc (kiloparsec) ichnoscale for detecting alien signals.

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The landing is the tricky part.

Mars is a difficult planet to land on. It has just enough of an atmosphere to be a problem, about 1% the pressure of Earth’s atmosphere. This is thin so provides much less breaking (but still useful) to slow the craft, but thick enough to produce dust storms and other menaces. Mars has 0.376 G surface gravity, which is a lot less than Earth but significantly more than the Moon. It is enough to make landing a challenge.

The Mars Perseverance Rover is set to land on Mars at 4pm Eastern time today (2/18/2021). This is perhaps the trickiest Mars probe landing yet, specifically because we are aiming for difficult terrain. Previous landers and rovers went for safe landing sites, rather than scientifically most interesting sites. Perseverance will have instruments to look for signs of ancient life, and so has to go where the looking is good, not necessarily where the landing is safe.

The landing procedure has been characterized by NASA as seven minutes of terror. The communication delay to Mars at this time will be 11 minutes, so it will all be over before we seen what happened. The landing procedure starts with the lander breaking away from the spacecraft. The lander has a heat shield which will slow the craft as it screams through the thin Martian atmosphere. Once the craft is slowed enough, it will deploy a parachute to further slow it down.

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Like many things in the universe, the complexity of reality defies our attempts at simple categorization or clean demarcation lines. One humorous example sometimes offered – is a taco a sandwich? But there are many serious challenges in categorization: What is a planet? There are reptiles that give birth to live young and two mammals that lay eggs. Disease classification in medicine is often a mess of blurry lines and statistical probabilities.

Along these lines – where is the edge of our solar system? I do think there is a reasonable answer here, but the solar system actually has several meaningful boundaries. The main part of the solar system, the part we all think of and which is represented in most models, contains the eight planets and everything within their orbits, including asteroids, comets, and dwarf planets. Let’s talk distance – Neptune is 4,609,592,833 km from the sun, or 30.8 au (astronomical units). An au is the distance from the sun to the Earth, which is about 150 million km. These are average distances. Neptune’s orbit is somewhat elliptical, 30.8 is its current and near maximal distance, but it gets as close as 28.8 au.

But clearly we would not end the solar system at Neptune. We have to include Pluto, which is the transition to the Kuiper belt and all the Kuiper belt objects (KBOs). The Kuiper belt is a ring in the plane of the solar system beginning at the orbit of Neptune, around 30 au, and extending out to about 1,000 au. So right there we have extended the size of the solar system in terms of distance from the sun by a factor of 33 times. The Kuiper belt is comprised of dwarf planets like Pluto, but also including Haumea, Makemake, and Eris. There are lots of bits of rick and ice, and likely many more dwarf planets and smaller planetoids to be discovered. Astronomers recently confirmed the orbit of the farthest confirmed individual object in the Kuiper belt, called Farfarout, with a highly elliptical orbit that takes it out to 135 au.

We can also consider the sun’s influence on surrounding space. The sun has a powerful magnetic field, which is called the heliosphere. The edge of its influence is called the heliopause, beyond this point interstellar conditions prevail and there is no longer any influence from the heliosphere. The heliopause is about 123 au from the sun.

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I like to keep my mental model of the universe updated as much as possible. One of the things I learned not too long ago about the universe is the frequency of “rogue” planets. These are planets that are not bound to any star. The dominant current theory is that most of these rogue planets formed in stellar discs just like bound planets but then at some point in their history interacted with another large body and were flung out of their stellar system. In fact, most planets that form in a stellar system may have unstable orbits that will doom them over the cosmological short term to either collide with another body, fall into their parent star, or get flung out of the system. Only those planets with stable orbits remain, and so eventually systems will settle down with a relatively few planets in stable orbits.

How many rogue planets (also called FFPs for free floating planet) are there in the Milky Way? We don’t know, and estimates vary wildly. The most cited recent estimate based upon computer simulations suggest that there are at least 50 billion rogue planets in the Milky Way. This is not much, actually, as there are between 100 and 400 billion stars, so that’s less than on per star. Still, that is a lot of rogue planets wandering the vast darkness of interstellar space. But other estimates of the number of rogue planets are much higher. At the high end one estimate put the number at 100,000 times greater than the number of stars. That seems implausible. But it suggests the number could be much higher than 50 billion. Even if on average there are several rogue planets for each stellar system, that would create hundreds of billions of such planets in our galaxy. And we have to include all the dead stars whose planets are still roaming interstellar space.

As an aside, any such estimate has to include a size parameter – how many FFPs of size greater than or equal to X. The estimates above use Pluto as the lower end of the size range. The lower you go, however, the greater the number.

What we need is some observational data to supplement the theoretical simulations. If we can do a survey looking for rogue planets then the number of such worlds we find can be plugged into our simulations to come up with better estimates for the total number. But how, you may be wondering (if you don’t know already) can we find a cold dark planet at the vast distances of interstellar space? Right now we find exoplanets by using a few methods. One method is the transit methods – we look at the dip in light output from a star as a planet passes in front of it. Another is the wobble method – we look for the wobble in the path of a star that indicates it is being tugged by a large planet going around it in its orbit. We might also be able to directly image a planet by looking at the reflected light off of it and removing the glare from the parent star. None of these methods will work for a rogue planet.

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