By Duncan Steel 19/09/2019


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The discovery of a true interstellar comet – a comet passing through the solar system having arrived, presumably, after having been thrown out of some other planetary system orbiting another star – re-opens a long-debated question in science: is life unique to Earth, or is it common in the galaxy? The panspermia hypothesis holds that life is common in the universe, and has been spread from planetary system to planetary system through the agency of interstellar comets. That idea could hold little sway so long as we knew of no such objects, despite observing thousands of comets gravitationally bound to the Sun. Now, though, an interstellar comet has been found, and it will undergo intense scrutiny as it barrels through the solar system over the next twelve months. 


Update, 2019 September 25: As I predicted, this interstellar comet has now been re-designated as 2I/Borisov. See this press release from the International Astronomical Union.


In a blog post last Friday I gave some initial information about the discovery of a comet (C/2019 Q4 Borisov) passing through the solar system, having come from interstellar space. I later appended some brief updates to that post, and also gave an extensive answer to a question posed with regard to the arrival direction of this comet (in terms of its direction of motion within our galaxy, the Milky Way). Now I provide an update covering some of the information that has been gained about this comet, and ideas regarding its origin, over the past five or six days.

The reader might ask: what’s the big deal? There are several big deals involved here. One aspect is limited to the astronomy involved, and that was mostly what I discussed in the previous post, and will continue with by-and-large here. Another aspect – and one that surely will be of wide appeal to the general public as well as many scientists – is the significance of the discovery of such a comet with regard to concepts of how life began on Earth… and so I will give a little bit of background on this, before proceeding with more information specific to this comet.


Comets and life on Earth are thought to be connected in a variety of ways, of which I might mention a few. First, when it first formed in the proto-solar nebula, our planet was extremely hot, indeed molten, and it would not be expected that water would survive as Earth agglomerated through the collision of many smaller planetesimals, arriving in hypervelocity impacts and releasing vast amounts of energy. Water must have come later, and it is a generally-accepted view amongst planetary scientists that the water – the sine qua non of life on Earth – was delivered later by cometary impacts, tiny fractions of the water surviving dissociation in each explosive arrival. This is a research topic in which I have myself been involved a great deal over the decades, for example contributing a chapter entitled Cometary impacts on the biosphere to a book entitled Comets and the origin and evolution of life

Not only water may have been supplied to Earth by comets, but also we know that comets (and some asteroids, and a rare type of meteorite known as a carbonaceous chondrite) contain large fractions of organic chemicals: not only methane and ethane, but also ethyl alcohol, and a wide variety of more complex organics and hydrocarbons. Such chemicals would not have been stable on Earth during its ferocious beginnings, and so again the finger is pointed at comets being the vectors bringing the essential ingredients of life to our planet.

In the above (delivery of water and organics to Earth after it had cooled adequately for such molecules to avoid pyrolytic destruction) the era in question is early in the planet’s 4.5 billion year history, perhaps several hundred million years post-formation. We have reasons to believe that an intense, ongoing series of major impacts by comets and asteroids (termed the Late Heavy Bombardment) continued through the first 700 million years, with the earliest surviving rocks and the first signs of life being dated to about 3.8 billion years ago.

A third way in which comets (and asteroids) are connected with the evolution of life on Earth is through massive impacts interrupting the evolution of polycellular life after the emergence of the Ediacaran biota around 600 million years ago, followed by the Cambrian explosion starting at about 540 million years ago. Across the intervening half-billion-years-plus there have been several mass extinction events in which large fractions of the fauna and flora have been obliterated, and these have in several cases been linked to large comet or asteroid impacts. The best-known example, it hardly seems necessary to mention, is the demise of the dinosaurs around 65 million years ago.

A fourth concept involving comets or asteroids striking Earth entails rocks being thrown off our planet in such huge explosions, perhaps carrying viable spores of some description to other planets. This scenario is usually termed transpermia. We know that rocks are transferred to Earth from both the Moon and Mars as lunar and martian meteorites (and in a blog post last December I wrote about a small sample of a martian meteorite that I own); the question of whether microbial life might be naturally transported from one planet to another within such rocks is a matter of considerable scientific debate, with analogue rocks having been exposed to space on missions to the International Space Station in order to determine whether the microbes inserted therein can still be viable at least over the weeks and months of such experiments. It has been suggested by some that the conditions on early Mars might have been more conducive to the origination of life than the Earth (for reasons connected with the matters discussed in paragraphs above), and that in fact we might all be martians. That is, primordial life got going on Mars, then was transferred here, but the loss of water and extensive atmosphere on the Red Planet later caused that cradle of life to become sterile, whilst it thrived on Earth once our planet had cooled down and accumulated sufficient water and organics to support monocellular life.

The trajectory of interstellar comet C/2019 Q4 (Borisov) through the solar system.

A fifth link between comets and life on Earth is the one that is of major interest here. The word transpermia was invented as a variation on an earlier term, panspermia. Panspermia is the idea that life may have spread, perhaps from a single origination somewhere, throughout the universe (or at least our galaxy) by being carried between planetary systems orbiting other stars through some vessel able to contain and conserve microbial life, likely in a dormant state. The favourite such vessels are large comets.

Whilst this hypothesis might seem to be just a way of avoiding the problem of working out how life started on Earth (simply by saying that it got started somewhere else and then was delivered here), obviously we would still need to identify how it got started anywhere. Nevertheless, there are reasons for the panspermia concept having some attractive aspects, in terms of scientific considerations. One such consideration is this: as aforementioned, the Earth was formed a little over 4.5 billion years ago, along with the Sun and the rest of the solar system. That, though, is only about one-third of the age of the universe. Whilst it is true that the production of heavier elements required stars to go through their evolutionary stages (because in the beginning there was light, and then there was hydrogen and helium, and later heavier elements like carbon and nitrogen and oxygen were gradually formed by nucleosynthesis – fusion reactions in stellar cores – and later still elements heavier than iron were produced in supernova explosions), actually stars far more massive than the Sun evolve much quicker. It might have needed a billion years before the building blocks needed to produce rocky planets (elements like silicon, and sulphur, and metals) were available so as to agglomerate from collapsing interstellar clouds of gas and dust, forming new stars and associated planets, but still there would have been perhaps eight billion years during which some stars had planetary systems prior to the eventual formation of the Sun and the solar system.

The panspermia hypothesis holds, then, that such planetary systems orbiting distant stars might have spawned life, and that life may have been carried out of those systems within comets set on wandering paths through our galaxy, just as we have observed comets being thrown out of the solar system, by the massive gravity of Jupiter in particular.

The above, naturally, was indeed a hypothetical scenario limited by the lack of any detection of a comet coming from interstellar space, and therefore not a member of the solar system. But at the end of August we found one. The previous interstellar object spotted (‘Oumuamua, discovered in October 2017) appears to have been an asteroid, meaning that it is likely rocky and may not have the internal environment to harbour microbes travelling for millions of years. Object C/2019 Q4 (Borisov), though, is assuredly a comet: a dirty snowball, full of deep-frozen water and many other ‘ices’ (frozen molecules such as methane, argon, carbon dioxide, ammonia) and organic chemicals. This comet, the reasoning goes, could carry extraterrestrial life from one star system to another.


The above, I hope, gives some context, indicating the significance of the discovery of C/2019 Q4 (Borisov). Now let’s turn to what we know about it so far.

First, it’s nothing spectacular to see. To detect it, one needs a reasonably-powerful telescope (the smallest I have heard of so far had an aperture of 15 centimetres) and a CCD detector. The image below was gathered by the discoverer, Gennady Borisov.

CCD image frame showing C/2019 Q4 (Borisov) – note the two lines indicating the faint comet – actually obtained by Gennady Borisov himself.

Currently the comet is in the northern sky, but will gradually move southwards and so will be best observed by southern hemisphere astronomers from the start of 2020. That is also when it would be anticipated to be intrinsically brightest: the comet passes perihelion around December 9th, and typically comets are liberating more vapour and dust shortly post-perihelion, so that January next year might be a good bet. This is especially the case since the comet will be the closest it comes to Earth at around that time.

If you would like to see where the comet is now, and where it will be in the future, or input your own location for planning observations, go to this wonderful website.

As I explained in my initial blog post about this comet, its scientific importance stems from its interstellar trajectory. The orbit and ephemeris determinations for C/2019 Q4 have been improved, as is to be expected, with the gathering of more astrometric observations by a small army of astronomers, most of them keen (and well-equipped) amateurs. The latest definitive information with regard to the orbit I have seen is available here.

Update 2019/09/19, 16:00 NZST: an improved orbit was published about 13.5 hours ago. The eccentricity evaluation is now e ≅ 3.4, and the perihelion distance q ≅ 2.02 AU. (Those two are linked: an increase in the determined value of q results in an increase in e, for a hyperbolic orbit.) 

If one looks at the ephemeris given at the bottom of this webpage, or this one, it is seen that positions are given for the comet a couple of weeks before it was actually discovered. How come? Well, it is possible that somewhere an observer scouring the heavens may have recorded the comet at that time, but not recognised it. If a position were identified from, say, mid-August, then the observation arc would immediately be extended and so a better orbital evaluation might be made. We term such identifications precoveries: a recovery of a position from prior to the formal discovery.

Another source of orbital information for this comet is the NASA-Jet Propulsion Laboratory Small Body Database. The three orbital diagrams I show in this blog post were generated using that website. If you would like to shift around and get views of the solar system and this comet’s path from different directions, simply click here. In passing I thank the US taxpayer for making facilities like this freely available.


News media elsewhere are buzzing with stories about this comet, and I will give links to a few of the better ones below. With a large number of astronomically-expert editors, the Wikipedia page for this comet is a good source of near-up-to-date information.

Here are a few links, then:
European Space Agency (September 12th).
Scientific American (September 13th).
Bad Astronomy/Phil Plait (September 13th: so old-ish, but highly-recommended by me).
Science News (September 16th).
Space.com (September 16th).
BBC (September 17th).
Popular Science (September 17th).
The Verge (September 17th).
The Atlantic (September 18th).


Let me turn next to some early scientific results. While invaluable astrometric positions have been gained by amateur astronomers using small apertures, to be able to gather enough light to split it across the visible spectrum (and into the near-infrared) requires large telescopes. We are talking eight- or ten-metre reflectors here.

At the Gemini Observatory in Hawai’i, astronomers obtained a multi-colour composite by imaging through two different filters; this showed the coma and fledgling tail to be basically grey (no surprise there: it’s mostly water vapour). That composite image has been carried by many media websites, so I will not repeat it here.

That image was also used on this Astronomy magazine webpage, which announced other results indicating that the solid cometary nucleus is “very red”. Similarly, that is not in itself a surprise, because comets in general have reddish hues due to the preponderance of thick tarry organics left on their surfaces as the lighter organic (alkanes, alkenes, alcohols etc.) and inorganic (water, CO, CO2, NH3, Ar) fractions sublimate under solar radiation heating. In interstellar space one might anticipate a surface coating of heavy organics also to build up as the cometary nucleus is bombarded by UV, X-rays, charged particles, and dust grains. This redness of the nucleus has been confirmed by other data collection, with specific numbers reported in this preprint.

A word of caution: when astronomers say that an asteroid or comet is “very red”, you should not imagine that it would look scarlet to the eye, like Little Red Riding Hood’s cape. What is meant is that the albedo or reflectivity is higher at wavelengths between 600 and 700 nm (the red end of the visible spectrum) rather than between 400 and 500 nm (the violet-blue end of the spectrum). In fact cometary nuclei in general have very low albedos, reflecting less than four per cent of the sunlight that hits them, making them darker than a lump of coal or a newly-tarmacked street. To the eye one might imagine a typical cometary nucleus as being black, black, black with a slight hint of brown.

This increased reflectivity at the red end of the spectrum is shown in the following plot, obtained using the 10.4-metre aperture Gran Telescopio Canarias.

Left: image of C/2019 Q4 with a length scale indicated for the situation at the comet’s vast distance from Earth, about 500 million kilometres at present. At right: the reflected spectrum of the comet’s solid nucleus, with a comparison against the ranges displayed by D-type asteroids (many of which are thought to be extinct or dormant comets).

So, the reflection spectrum of this interstellar comet appears similar to D-type asteroids, and also solar system comets (of which we have observed thousands). The best guess is that the surface composition of the comet is a mixture of dust and silicates, held together by tarry deposits left behind as the more-volatile fraction has been lost.


If this interstellar comet is so important, and we don’t know when another will come along, wouldn’t it be good if we could send a spacecraft to investigate it close-up? Perhaps we might even find evidence of life from a distant planetary system (though I very much doubt it).

In fact we have had a long-term perceived desire to send a space mission to a long-period comet, those comprising most of the cometary flux through the inner solar system. These are comets that appear once, without warning, and have orbits that are near-parabolic. Without any advance notice, we would need to have a probe ready for launch in order to intercept such a beast, found perhaps a year or 18 months prior to a rendezvous opportunity.

Back in 1995 I suggested just such a thing, at a conference on the island of Vulcano which we entitled Beginning the Spaceguard Survey. As I knew then, and several others present pointed out, such an idea was never going to fly. No space agency was going to approve the spending of around half a billion dollars on readying a satellite to be sent to a target that had not yet been discovered. The spaceprobe might need to wait for a decade or more until an accessible comet was spotted, and by then the technology would be almost out of date; that is, you would not build the probe in the same way again, because better methods and electronics would have become available.

Regardless, it would be wonderful if it were feasible to send a mission to this interstellar comet, and several researchers have already reported on this (im)possibility. Take a look here, and here.


The situation in the astronomical world just now was nicely summarised in the Astronomy magazine article previously mentioned: “In the meantime, astronomers are rushing to put together proposals to use major telescopes to observe the comet in greater detail in the weeks to come, hoping to measure things like its composition, size and rotation.

What will be discovered, we know not at this stage, but it is certain to be illuminating. Over the next twelve months you can be sure that you will be hearing more about interstellar comet Borisov.

Another view of the interstellar comet’s path. Don’t worry, it passes nowhere near Earth.