Dark matter is one of the greatest current scientific mysteries. It’s a fascinating story playing out in real time, although over years, so you have to be patient. Future generations might be able to binge the dark matter show, but not us. We have to wait for each episode to drop. Another episode did just drop, in the form of an analysis of the massive relic galaxy NGC 1277, but let’s get caught up before we watch this episode.

The term “dark matter” was coined by astronomer Fritz Zwicky in 1933 as one possible explanation for the rotation of the Coma Galaxy Cluster. The galaxies were essentially moving too quickly, implying that there was more gravity (and hence more matter) present in the cluster than was observed. This matter could not be seen, therefore it was dark. The notion was a mere footnote, however, until the 1970s when astronomer Vera Rubin analyzed the rotation curves of many individual galaxies. She found that galaxies were rotating too quickly. The stars should be flying apart because there was insufficient gravity to hold them together (or alternatively they should be rotating more slowly). There must be more gravity that can be seen. The notion of dark matter was therefore solidified, and has been a matter of debate ever since.

Half a century after Rubin confirmed the existence of dark matter, we still don’t know what it is. It must be some kind of particle that does not interact much with other stuff in the universe, does not give off or reflect radiation, but possesses significant mass and therefore gravity. There are candidate particles, such as wimps (weakly interacting massive particles), MACHOs (massive astrophysical compact halo object), axions (particles with a tiny amount of mass but could be very common) or perhaps even several particles currently not accounted for in the standard model of particle physics.

This is one of the exciting things about dark matter – when we figure out what dark matter is, it could break the standard model, pointing the way to a new and deeper understanding of physics. But how certain are we that dark matter exists? To a degree the existence of dark matter is an argument from ignorance – it is a placeholder filling in a gap in our knowledge. We can only infer its existence because we cannot explain with our current models of gravity how stuff is moving in the universe. Perhaps our current models of gravity are wrong?

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It’s always exciting when a scientific institution announces that they are going to make an announcement. Earlier this week we were told that there was going to be a major announcement today (June 29th) regarding a gravitational wave discovery. The goal of the pre-announcement is to generate buzz and media attention, although I almost always find the reveal to be disappointing. I guess we are too programmed by movie plotlines where such reveals are truly earthshattering. So I have learned to moderate my expectations (a generally good strategy to avoid disappointment).

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) team released five papers late last night in the Astrophysical Journal Letters – I think my moderated expectations were pretty much on target. This is awesomely cool science, but didn’t shatter my world. I can see why the scientists were so excited, however. This was the culmination of 15 years of investigation. The bottom line discovery is that spacetime is constantly rippling, in line with Einstein’s predictions of General Relativity. Let’s get into the details.

Gravitational wave astronomy is a new window onto the universe. Most of the recent news has been made by the Laser Interferometer Gravitational-wave Observatory (LIGO). This is a large instrument, with two powerful lasers at right angles to each other, with each arm about 4 km long firing through a vacuum pipeline. Where the lasers cross they create an interference pattern. The slightest disturbance in the lasers can change the interference, and therefore it is a very sensitive detector. The primary challenge is isolating LIGO from background noise and filtering it out. What is left is a signal produced by gravitational waves, ripples in spacetime created by massive gravitational events. LIGO is able to detect high frequency gravitational waves formed by the collision of black holes and/or neutron stars with each other.

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What will be the ultimate fate of our universe? There are a number of theories and possibilities, but at present the most likely scenario seems to be that the universe will continue to expand, most mass will eventually find its way into a black hole, and those black holes will slowly evaporate into Hawking Radiation, resulting in what is called the “heat death” of the universe. Don’t worry, this will likely take 1.7×10¹⁰⁶ years, so we got some time.

But what about objects, like stellar remnants, that are not black holes? Will the ultimate fate of the universe still contain some neutron stars and cold white dwarfs that managed to never get sucked up by a black hole? To answer this question we have to back up a bit and talk about Hawking Radiation.

Stephen Hawking famously proposed this idea in 1975 – he was asked if black holes have a temperature, and that sent him down another type of hole until Hawking Radiation popped out as the answer. But what is Hawking Radiation? The conventional answer is that the vacuum of space isn’t really nothing, it still contains the quantum fields that make up spacetime. Those quantum field do not have to have zero energy, and so occasionally virtual particles will pop into existence, always in pairs with opposite properties (like opposite charge and spin), and then they join back together, cancelling each other out. But at the event horizon of black holes, the distance at which light can just barely escape the black hole’s gravity, a virtual pair might occur where one particle gets sucked into the black hole and the other escapes. The escaping particle is Hawking Radiation. It carries away a little mass from the black hole, causing it to glow slightly and evaporate very slowly. This evaporation gets quicker as the black hole becomes less massive, until eventually it explodes in gamma radiation.

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ESA’s Gaia orbital telescope has recently discovered two new black holes. This, in itself, is not surprising, as that is Gaia’s mission – to precisely map the three-dimensional position of two billion objects in our galaxy, using three separate instruments. The process is called astrometry, and the goal is to produce a highly accurate map of the galaxy. The two new black holes, Gaia BH1 and Gaia BH2, have two features that make them noteworthy. The first is that these are the two closest blackholes to the Earth every discovered, at 1560 and 3800 light years away (on galactic terms, that’s close). More important, however, is that they represent a new type of black hole.

Black holes are the most massive single objects in the universe. They are made from stellar remnants that have sufficient mass that the inward force of gravity is greater than any outward force of the matter itself. If the stellar remnant is >2.16 solar masses, it will become a black hole. Less than that, down to about 1.4 solar masses, and you have a neutron star. Stars of 20 solar masses or more are large enough to leave a stellar remnant behind after they nova to create a black hole. Black holes can also be formed when two neutron stars collide, the resulting mass being greater than the 2.16 limit. Or, a neutron star can have a close companion and it can gravitationally draw mass from the companion star until it reaches the black hole limit.

There are also supermassive black holes, usually formed at the center of galaxies from gobbling up more and more stars. The most massive discovered so far is 66 billion solar masses.

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NASA recently discovered a 50 meter wide asteroid whose orbit will come close to Earth. They estimate a close approach in 2046, which will likely bring the asteroid within 1.1 million miles of the Earth, about four times the distance of the moon. However, there is always uncertainty in calculating orbits, and the farther into the future you try to project their path, the more uncertainty there is. At this point in time NASA estimates a one in 560 chance that the asteroid, dubbed 2023 DW, will hit the Earth in 2046.

Orbits are calculated through multiple observations of the object along its orbit. We have to see how it is moving, and the longer the observation the greater the precision. For recently discovered objects, like this asteroid, there is more uncertainty in the orbital calculations, which is why NASA cannot completely rule out an impact. Because it is a near-Earth object, however, they will continue to make observations, refining their calculated orbit, and reducing the uncertainty.

Interestingly, the current scale for designating the risk of an object hitting the Earth, the Palermo scale, is not based on a simple percentage probability, but on the probability relative to the background rate of impacts. A Palermo scale of 0 means that the chance of a particular object hitting the Earth is no different than the background rate of impacts. The scale is also logarithmic, so a Palermo rating of 1 means a chance of impact 10 times the background rate, 2 is 100 times. 2023 DW has a Palermo rating of -2.17.

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I recently received an e-mail question from an SGU listener about the speed of gravity. They were questioning a statement they heard by Neil DeGrasse Tyson that if the sun were magically plucked from existence, the Earth would not feel the effects for 8 minutes and 20 seconds – the time it takes for light to travel from the sun to the Earth. This blew their mind, writing: “That statement doesn’t make sense to me. What DeGrasse is saying is that we don’t actually orbit the sun but a point in space where the sun was 8 minutes 20s ago.”

Actually, that’s exactly right. He did get it. We see the sun as it was 8 minutes 20 seconds ago. We also feel the sun as it was at that time – in every way. The speed of light is more than the maximal relative velocity that energy travels through the universe, it is the speed at which reality propagates throughout the universe. No effect can exceed the speed of light – not information, matter, energy, or force.

We can use the General Relativity conception of gravity, that matter curves space time and that matter and energy travel in a straight line through curved space. You have likely seen the typical graphical representation of this, with a heavy object like a planet or star distorting a grid of spacetime like a heavy ball resting on a stretched piece of fabric. This is actually just a conceptual aid, not an accurate depiction. It is missing one dimension – space is represented as a two-dimensional fabric stretching into a third dimension. You need to add a dimension to represent reality, which is three dimensions. Some experts say that three dimensional space is being curved in a fourth dimension, but others say you can work the math to describe the curvature of 3D space without invoking a 4th spacial dimension. I’m not sure what the current consensus is – I’ll have to look deeper when I have time (if anyone has a good reference, please share it in the comments).

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Scientists love mysteries, because that is where new discoveries lay. It is nice to find evidence consistent with existing theories, providing further confirmation, but it’s exciting to find evidence that cannot be explained with existing theories. Astronomers may have found such a mystery in the dwarf planet Quaoar – it has a ring where one shouldn’t be.

When we think of planetary rings we of course think first of Saturn, which has by far the largest and most impressive ring system in the solar system. But all the gas giants have their own rings, including Jupiter, Uranus, and Neptune. None of the smaller rocky planets have detectable rings. This is likely not a coincidence and relates to how rings form and how long they will last. But smaller bodies can have rings. Saturn’s moon Rhea may have a faint ring of its own. There is even an asteroid, Chariklo, which has two faint rings.

Rings are basically linear clouds of dust, ice, and other material that spreads out in an orbit around a planetary body. They form in one of two ways. For large planets like Saturn, if a moon’s orbit decays to the point where it gets within the Roche limit (the point at which tidal forces are so great they tear apart any large object), then the moon will break apart into debris that forms a ring. But anything that spreads dust around a planet can also feed a ring. For example, when meteorites hit the moons of Jupiter they throw up dust which can feed its faint ring system. Because the material that forms rings are close to their host planet they also tend to slowly rain down to the surface, so they have a limited lifetime. Saturn will eventually lose its beautiful rings. But new rings may also form. When Phobos gets too close to Mars one day it will break up and become a Martian ring system.

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The universe is a big place, and with a variety of powerful telescopes astronomers can see it all (at least the visible universe). This means we can potentially see extremely rare events. One such rare event is a kilonova, a type of nova that results from two neutron stars or neutron star and a black hole merging together.  Even more rare – astronomers may have recently identified a system that will one day create a kilonova, in the Milky Way. They estimate that there may be only ten such systems in the entire galaxy.

To quickly review, the term “nova” means “new” in latin, because they were seen as new temporary stars that appear in the sky, last a few weeks and then disappear. A basic nova results from a white dwarf star with a close companion star. Over time the white dwarf may gravitationally pull hydrogen from the outer atmosphere of the companion star, until enough of it collects on its surface to kickstart runaway nuclear fusion, resulting in a significant brightening of the white dwarf. This is generally not explosive or destructive, and a single white dwarf can potentially go through many such novae.

There are a couple of types of supernova. The classic type of supernova results from the collapse of the core of a sun that had burned through its fuel. This rapid gravitational collapse results in an explosion of radiation, throwing off any remaining outer material and leaving behind a stellar remnant – neutron star or black hole.  The type 1a supernova results when a white dwarf draws enough material from a companion that it starts to have fusion in its core which rapidly burns through its fuel and collapses to form a supernova. A hypernova results from a very large star (30 or more solar masses) undergoing core collapse and leaving behind a black hole. (I’m skipping a lot of detail but this is basically how it breaks down.)

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When I was a child I loved looking up at the night sky and seeing thousands of stars. I especially loved seeing the disc of the Milky Way spreading across the sky. I can’t remember when this first dawned on me, but as an adult I can no longer do this. When I look up into the sky, even on a clear night, I can still see lots of stars, although not as many, and I can’t make out the Milky Way. It’s simply not visible.

The problem of light pollution was also brought home to me when I visited Australia. Seeing the southern sky was always an item on my bucket list – to see the Southern Cross, the Magellanic clouds, and Alpha Centauri. I have been in the southern hemisphere three times. The first two times I never got a look at the night sky. So on the third trip I made it a point to go somewhere where I would get a good view of the night sky. I had to drive over an hour from Christchurch to get that view. It was magical, and definitely worth the effort, but it really demonstrated for me the extent of light pollution.

A recent study published in Science found that:

Trends in the data showed that the average night sky got brighter by 9.6% per year from 2011 to 2022, which is equivalent to doubling the sky brightness every 8 years.

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I’m still waiting. Since we developed the technology to detect exoplanets – planets orbiting other stars – I have been tracking those exoplanets that are the most Earth-like. That term, “Earth-like”, is used quite a bit in science news reporting about exoplanets, but very loosely, in my opinion. I’m still waiting for an exoplanet discovery that is fully Earth-like.

This happened again just recently with the discovery of a second planet in the TOI 700 system that is “Earth-sized” (that’s more accurate than saying “Earth-like”). Unfortunately, TOI 700 is a red dwarf, which means the two Earth-sized planets technically in their habitable zone are also likely tidally locked. Further, red dwarfs are unstable compared to orange or yellow stars and may strip the atmospheres from any planets close enough to be in the habitable zone.

Before I review the best candidates – what makes an exoplanet “Earth-like”. The two criteria that seemed to be used by most reporting is that they are small rocky planets in their generously defined habitable zone. Often the term is applied to so-called “super-Earths” which are more massive than Earth but less massive than ice giants – basically anywhere between Earth and Neptune. It seems astronomers agree on an upper limit of mass of 10 Earth masses, but disagree on the lower limit (anywhere from >1 to 5). They should just pick a number. I think something like 2 Earth masses is reasonable, but perhaps it’s better to use surface gravity. We can use the formula a=GM/R^2 to determine surface gravity. So, for example (if I did the math right) a planet with 2 times Earth’s mass and 1.2 times the radius would have a surface gravity of 1.38 G. What about the lower limit? I would suggest somewhat larger than Mars – we could make an arbitrary cutoff of 0.5 G surface gravity.

The habitable zone is the distance from the parent star where it is possible to have liquid water on the surface. But there are lots of other variables here as well, mainly relating to the atmosphere. Venus, for example, is technically in our sun’s habitable zone, as is Mars, but neither are habitable. If Mars had more atmosphere and Venus less, however, they could have a survivable environment.

I think exoplanets around red dwarfs at this point need to not count as “Earth-like” even if size and temperature are in the range. They would have to be very close to their parent star, which means they are likely tidally locked (in itself not a deal-killer) and likely don’t have much of an atmosphere. There may be exceptions to this, and there are lots of red dwarfs so we may ultimately find some special planets around red dwarfs with life, but for now it is so unlikely they should simply not be on the list. Orange and yellow suns are the best candidates. Larger and brighter than yellow and the lifespan of the star becomes too short, but still may be a candidate with the right conditions for people to settle. Moons of gas giants are another possibility, but the variables get more complicated.

So to be truly Earth-like we would want a planet with a surface gravity somewhere between 0.5 and 1.3 that of Earth, that is small and rocky, and that orbits an orange to yellow star in the habitable zone. But there are other things that can go wrong with any candidate world. We also need to consider what question, exactly, we are asking. Are we interested in worlds we could one day settle? That would mean they also need to be very close, within 20 light years or so. Are we looking for a world that is already harboring life, and how much time would we want for that life to have had to evolve? This is important if we are looking for technological civilizations.

Here is a list of the ten most Earth-like exoplanets discovered so far. None really meet my criteria. Most orbit red dwarfs.  Some are super-Earths.

It’s too early to be discouraged, however. Some of our planet-finding techniques favor larger planets, or ones very close to their host stars. It is therefore harder and takes longer to discover small rocky worlds at Earth like distance from their stars. Astronomers estimate there are 300 million to 40 billion Earth-like planets in the Milky way. That is still a huge variance, but taking an average figure, that’s a lot. That number will be refined as we search more stellar systems for their planets. There are about 100 billion stars in the Milky way, but many of them are in multi-star systems. Astronomers estimate that 1 in 5 systems have at least one “Earth-like” planet, but again, I wonder what definition they are using. Most of these are likely red dwarfs (because most stars are red dwarfs).

Hopefully they will nail down these numbers with higher confidence in the near future. It would also be nice to complete a survey of all the closest stars to our system. Meanwhile I will keep tracking new discoveries.