Toby Ord covers 'Asteroids and Comets' and 'Stellar Explosions' in The Precipice. But I thought it would be useful to provide an up-to-date and exhaustive list of all cosmic threats. I'm defining cosmic threat here as any existential risk potentially arising from space. I think this list may be useful for 3 main reasons:

1. New cosmic threats are discovered frequently. So it's plausible that future cause areas could pop out of this space. I think that keeping an eye on it should help identify areas that may need research. Though it should be noted that some of the risks are totally impossible to protect against at this point (e.g. a rogue planet entering our solar system).
2. Putting all of the cosmic threats together in one place could reveal that cosmic threats are more important than previously thought, or provide a good intro for someone interested in working in this space.
3. There is momentum in existential risk reduction from outer space, with great powers (Russia, USA, China, India, Europe) already collaborating on asteroid impact risk. So harnessing that momentum to tackle some more of the risks on this list could be really tractable and may lead to collaboration on other x-risks like AI, biotech and nuclear. 

I will list each cosmic threat, provide a brief explanation, and find the best evidence I can to provide severity and probability estimates for each. Enjoy :)

I'll use this format:

Cosmic Threat [Severity of worst case scenario /10] [Probability of a scenario as bad as that occurring in the next 100 years] Explanation of threat

Explanation of rationale and approach

Severity estimates

For the severity, 10 is the extinction of all intelligent life on Earth, and 0 is a fart in the wind. It was difficult to pin down one number for threats with multiple outcomes (e.g. asteroids have different sizes). So the severity estimates are for the worst-case scenarios for each cosmic threat, and the probability estimate corresponds to an event occurring that is as severe as that scenario. 

Probability estimates

Probabilities are presented as % chance of a worst case scenario-type event occurring in the next 100 years. I have taken probabilities from the literature and converted values to normalise them as a probability of their occurrence within the next 100 years (as a %). This isn't a perfect way to do it, but I prioritised getting a general understanding of their probability, rather than numbers that are hard to imagine. When the severity or likelihood is unclear or not researched well enough, I've written 'unknown'.

I'm trying my best to ignore reasoning along the lines of "if it hasn't happened before, then it very likely won't happen ever or is extremely rare" because of the anthropic principle. Our view of past events on Earth is biased towards a world that has allowed humanity to evolve, which likely required a few billion years of stable-ish conditions. So it is likely that we have just been lucky in the past, where no cosmic threats have disturbed Earth's habitability so extremely as to set back life's evolution by billions of years (not even the worst mass extinction ever at the Permian-Triassic boundary did this, as reptiles survived). 

An Exhaustive List of Cosmic Threats

Format: 

Cosmic Threat [Severity of worst case scenario /10] [Probability of a scenario as bad as that occurring in the next 100 years] Explanation of threat

Solar flares [4/10] [1%]. Electromagnetic radiation erupts from the surface of the sun. Solar flares occur fairly regularly and cause minor impacts, mainly on communications. A large solar flare has the potential to cause electrical grids to fail, damage satellites, disrupt radio signals, cause increased radiation influx, destroy data storage devices, cause navigation errors, and permanently damage scientific equipment. This threat is mainly managed by the World Meteorological Organisation (WMO) and COSPAR.

Super flares [8/10] [0.1%-0.16%] Super flares could lead to changes to Earth's surface temperature and the destruction of our ozone layer, but the predicted effects are still unknown. Some predict that space weather events like these have the potential to cause the extinction of many species and severe destruction to technology and global infrastructure. Resulting failures in nuclear threat detection or global panic could trigger x-risks. This threat is mainly managed by the World Meteorological Organisation (WMO) and COSPAR.

Supernova explosions [8/10] [0.001%]. According to this study, the closest stars that could pose a threat to humanity if they went supernova are IK Pegasi and Betelgeuse, but they are not expected to go supernova until 1.9 billion years and several million years, respectively. However, no systematic survey of all white dwarf stars (e.g. using GAIA data) has been conducted, and there may be others (I've adjusted the probability estimate based on that lack of research). The impact of a supernova explosion would be quite devastating to life on Earth, primarily due to damage to our ozone layer. This threat is mainly managed by ESAC in theory. 

Gamma-ray blasts [7/10] [0.000005%]. Gamma-ray bursts are more powerful than supernova explosions, and could potentially generate nitrogen oxides, which could rip large holes in our ozone layer. This could leave Earth exposed to UV rays for many years. This threat is mainly managed by ESAC.

Asteroid Impacts [10/10] [0.02%]. This has happened many times in Earth's past. Space missions such as DART, Hera, and NEOWISE reduce this probability as we are fairly well prepared to deflect asteroids. We have evidence that asteroids have the potential to cause extinction events through direct impact destruction, tsunamis, and giant ash clouds that block out the sun. This threat is mainly managed by SMPAG and IAWN.

Other Near-Earth Objects (NEOs) [6/10] [a: 0.003% b: 0.06%]. Other near-Earth objects (not necessarily asteroids - there are many smaller objects in the Solar System) smashing into Earth are probably quite common, we can tell from all the craters on the moon. Probability 'a' is the probability that an asteroid impacts and kills more than 1 million people. Probability 'b' is the probability that an asteroid hits and forms a crater larger than 1km in diameter. This threat is mainly managed by SMPAG and IAWN.

Conflict with intelligent alien life [10/10] [unknown]. Intelligent life exists on Earth, so it's possible it could exist elsewhere. Hopefully, they don't invade and kill us all. No one manages this threat seriously, the closest thing is SETI. 

Alien Technology [10/10] [unknown]. Conflict with alien technology is slightly more likely than direct conflict with intelligent alien life. Self-replicating robots may have been sent out throughout the galaxy, consuming the resources of a planet, replicating themselves, and moving on. This is theoretically possible, and the universe is a very large place. No one manages this threat seriously, the closest thing is SETI. 

Rogue Celestial Bodies [10/10] [0.002%]. Rogue celestial bodies include black holes, planets, and stars that are not gravitationally bound but wander around the galaxy. One of these celestial bodies could potentially enter our Solar System. There are many exciting possibilities from this point.. Earth could be knocked out of its orbit, sending us hurtling through open space. Another planet (or planetoid) could impact us (this has happened before and the ejecta formed the moon). Or if the rogue celestial body is exotic, we could be swallowed up by a black hole or burnt to a crisp by a second sun. Managing this threat is futile. 

Vacuum decay [10/10] [unknown]. Vacuum decay is a hypothetical scenario in which a more stable vacuum state exists than our current "false vacuum". A bubble of this true vacuum could form somewhere in the universe or in a high-energy particle accelerator. To quote Nick Bostrom, "This would result in an expanding bubble of total destruction that would sweep through the galaxy and beyond at the speed of light, tearing all matter apart as it proceeds.". Incredibly speculative, no one manages this threat from a cosmic perspective as far as I'm aware.

Magnetar flares [9/10] [unknown]. Similar to solar flares and gamma-ray bursts, except with less research into the probability of their occurrence. A magnetar is a neutron star with an extremely powerful magnetic field. It sends out high-energy electromagnetic radiation, like X-rays and gamma rays. This threat is mainly managed by ESAC in theory.

Alien Microbes [7/10] [<0.0001%]. Some have considered cancelling NASA and ESA's Mars Sample Return mission due to concerns about biosecurity. Termed back contamination in astrobiology (as opposed to forward contamination where we contaminate Mars samples with life from Earth), the idea is that bringing back extremely hardy life from Mars could either cause a pandemic or introduce a species capable of outcompeting microbes on Earth, undercutting ecosystems. This threat is managed by COSPAR and CETEX.

Pulsar beams [9/10] [unknown]. "A pulsar (from pulsating radio source) is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles." Pulsar beams hit Earth frequently and don't cause much harm, but if a pulsar were to be very close to us, it could wipe out our ozone layer and expose us to the radiation of space - it wouldn't be pretty. This threat is mainly managed by ESAC in theory.

Quasar ignition [10/10] [unknown]. Quasars form around black holes. "The inflow of gas into the black hole releases a tremendous amount of energy, and a quasar is born. The power output of the quasar dwarfs that of the surrounding galaxy and expels gas from the galaxy in what has been termed a galactic superwind" This "superwind" drives all gas away from the inner galaxy, and would certainly rip off our atmosphere, leaving us totally exposed. This threat is mainly managed by ESAC in theory. 

Cosmic Rays from Galactic Core Explosions [10/10] [unknown]. Hundreds of thousands of times more damaging than supernova explosions, these cosmic rays originate from the cores of galaxies. Some theorise that cosmic rays from other galaxies have already caused mass extinctions on Earth. Explosions from the core of the Milky Way likely make the inner galaxy uninhabitable, so large explosions may have catastrophic impacts on Earth. This threat is mainly managed by ESAC in theory. 

Some artificial existential risks from space..

Asteroid Orbit Interference [10/10]. In the event that humanity colonises the solar system or establishes significant space resource operations, it will become a lot easier to alter asteroid orbits. An asteroid orbit may accidentally be placed on a collision path with Earth through by messing up stable gravitational interactions by alteration or destruction of other asteroids, or by accidental collisions of resource extraction equipment and asteroids. An asteroid may also be re-directed towards Earth on purpose as an act of war. No international policy framework exists to govern space resource activities - these risks would most likely be handled by UNOOSA and national governments. 

Alien generation [10/10]. By colonising other planets, we generate additional societies in our solar system. These societies would live under very different conditions to us, so many diverge significantly in culture, religion, and potentially even biology. Additionally, due to the time delay in contacting these societies and distance, live communication would be impossible and determination of their war potential would be much harder than other societies on Earth. These factors increase the probability of conflict, so good governance is extremely important to make sure they don't start hurtling asteroids at us. 

and finally, some honourable mentions...

The heat death of the universe [10/10] [0%]. It's worth the inclusion as the heat death of the universe will eventually destroy everything. It's an inevitable outcome of the application of the first two laws of thermodynamics - entropy increases in an isolated system. So eventually the universe will reach a point where all energy is evenly distributed and not a lot will happen - certainly not life.

The increasing luminosity of the sun [10/10] [0%]. The sun's luminosity is increasing over time. In ~1 billion years the luminosity will be so high that the oceans will boil away and the Earth will no longer be habitable. Hopefully we'll be intergalactic by then!

Other unknown risks [unknown] [unknown]. The space (& physics) community is essentially discovering a new existential threat every few years, and we are likely to discover more. For example, we know very little about dark energy or how common life is in our universe. Most threats are extremely low probability, but maybe the next one won't be. 

If you have any feedback, edits, suggestions, or additions to the list, please comment or pm me. 

Acknowledgements: I extend my gratitude to Matt Allcock and Darryl Wright for their invaluable discussions and insights on these topics, which significantly contributed to the accuracy and direction of my research.