M51 And Its Planet

There simply is no more classic or charismatic clearly visible galaxy than M51, the Whirlpool Galaxy. If someone hears the word “galaxy” and it’s not about chocolate, I’d be prepared to bet that the image in their mind will be something like the above one, except for NGC 5915 shown on the right of the picture. Also, with all the Messier designations (the ones beginning with M), I tend to be reminded of motorways, but in the case of the M51 there is no such road in Britain, although there are examples elsewhere. In fact, the only two-digit motorway ending in 1 is the M61. I do not know why this is. But anyway . . .

It’s alleged that for the first time, a planet has been discovered in another galaxy, specifically this one, via an X-ray telescope. This is a tentative announcement. In the arm which sweeps down on the right and ends up around bottom left, near the top of the near-vertical portion, there is a star described as “Sun-like” orbiting a neutron star or black hole, designated as M51-ULS-1. The bright star is about twenty times the Sun’s mass, so in fact it really isn’t very Sun-like at all, being about the same mass as Rigel, which in fact means it may well no longer be there because such stars only last a few million years and the Whirlpool Galaxy is 28 million light years away. There was a three-hour period during which the X-ray emissions from this star dropped to zero, as detected by NASA’s Chandra X-Ray telescope in Cambridge, Massachusetts. The X-rays from this binary system are caused by the neutron star or black hole ripping plasma off the surface of the still luminous companion, heating it enough to cause major X-ray emission, and because the region of the system responsible for the X-rays is small, it can be totally eclipsed by the planet. The planet in question would be extremely inhospitable if the companion is a neutron star because of the extreme magnetic fields and X-rays. If X-rays can be detected from 28 million light years away, they’re bound to be pretty major at the distance of a planet actually orbiting within the system, so this is definitely not the abode of “life as we know it”. It’s likely to look something like this:

This is PSR 1257+12C, a pulsar planet (pulsars are neutron stars) orbiting a pulsar 2 300 light years away in Virgo. The aurora around the pole is caused by the intense radiation. This is a horrible place. The planet, if that’s what it is, was the result of searching nearly two gross possible places in other galaxies for one, some in M51, some in the Sombrero Hat Galaxy and some in the Pinwheel. It isn’t thought to be a gas cloud because of the way it blocked the radiation. These are all relatively famous galaxies for us on this tiny blue dot in this galaxy because of their appearance. This is the Pinwheel Galaxy:

By ESA/Hubble, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=36216331

. . . and this is the Sombrero Hat Galaxy:

Copyright status unknown – will be removed on request

The Pinwheel Galaxy is face-on to us, like the Whirlpool, and both are what’s known as “grand design” galaxies, whereas the Sombrero Hat galaxy clearly has something else going on where many of its stars are orbiting outside the main disc, possibly due to aging. I find this one the most visually appealing although I also suspect that whatever did that to it, or the arrangement of stars as such, makes it quite hostile to stable planetary systems.

If the planet in M51 is near a black hole, rather than being zonked by the radiation directly from a neutron star it will instead be zonked by the disc of gas around that object, but either way it’s not going to be nice and in a way the distinction is academic. These kinds of planets are the ones most likely to be detected by an X-ray observatory, and being anthropocentric one would perhaps wonder what the point of looking for inevitably extremely unpleasant star systems is, but then black holes and neutron stars are fascinating objects and we should probably just get over ourselves really. The Universe does not revolve around us, or any form of life, probably.

There will inevitably be loads of other planets in the Whirlpool Galaxy, just not ones detectable in that way from near Earth. Apparently, although I’ve never seen it myself, M51 is observable through binoculars on a clear night. It’s in one of the constellations whose names always remind me of pubs: the Hunting Dogs. The other one is the Fox And Goose. Officially known as Canes Venatici, the stars in this constellation are rather dim and it was named by Hevelius in the seventeenth Christian century rather than being traditional. Hevelius also named a number of other constellations, including Lynx, which is called that because it’s so dim only someone with eyes like a lynx would be able to see it, so it’s said. Because the Whirlpool Galaxy is only twenty to thirty million light years away, and I’ve just seen another estimate of thirty-seven million, it’s fairly large at eleven by seven minutes of arc, which is about a third the width of the Sun, but it has a magnitude of only 8.4, which is about five times too dim to be seen with the dark-adjusted eye (with good vision, unlike mine). One way of judging the distance of galaxies is by looking for Cepheid variables in them, whose pattern of brightening and dimming is related to their brightness, so the brightness of a Cepheid variable star in another galaxy can be established and compared to how bright it seems to be from here. I don’t know why the distance of M51 is not better known. NASA says it’s 28 million light years away though. Because it’s relatively prominent, it’s been used for a long time to study the structure of spiral galaxies, often with reference to our own because it used to be thought that the Milky Way was a similar shape, ignoring the complication of NGC 5915 that is, but nowadays it’s realised that our Galaxy is a barred spiral, a little like NGC 1300 but with more arms:

Another advantage of studying M51 is that it’s laid out flat at its angle to us, making it easier to see details.

I can’t track this down, but one of the notable things about the Whirlpool Galaxy is that some ‘Star Wars’ fans think it’s the Star Wars galaxy. Against this is the fact that the Spielberg ET species, who are from this galaxy, cameo briefly in one film. One fan-made map of that galaxy can be seen here and it is in fact a grand design spiral like M51 but there are no signs of NGC 5915 on that map. However, ET is three million light years from home rather than twenty-eight, and ET recognises a Yoda costume at Hallowe’en apparently (I haven’t seen ET), so it seems unlikely that it is this one and more likely that it’s the Triangulum Galaxy, 2.7 million light years away. But of course it’s all fantasy so who cares? I’m really, really not a fan of ‘Star Wars’ but that’s not for here.

NGC 5915 is rather structureless and fuzzy, and has probably been pulled out of shape by the tidal forces of the Whirlpool. For a while it was thought simply to be near it, or perhaps a little past it but in a similar line of sight, but closer examination reveals that it is in fact where it seems to be in relation to the larger galaxy. There is a dusty stream with a few stars around it stretching from one of M51’s arms to its companion. However, it is slightly behind the larger galaxy because the dust stream is in front of it from our perspective, outlined against the background of stars. There’s a black hole at its centre which is said to be “belching” gas, thereby stimulating the formation of stars along the route of the gas.

The Whirlpool Galaxy itself is apparently 60 000 light years across compared to our own 100 000 light year diameter and has a mass of 160 thousand million times that of the Sun. I presume this estimate is based on belief in non-baryonic dark matter and the rejection of modified Newtonian dynamics, but it would make it about forty percent as massive as the Milky Way either way. Three supernovæ have been detected in it since 1994, and in 2019 a red nova was detected and thought to be a supernova at first but the star was still there after the explosion, which is common with novæ, but this is not a normal nova but probably the collision of two stars, which has a distinctive red colour.

It would probably be instructive to compare M51 to the Milky Way in terms of suitability for life-bearing worlds. Certain aspects of our own galaxy are seen as unusual by some. It’s supposed to be dustier and unusually dim, for example. It’s also a barred spiral rather than the more whirlpooly appearance of M51. It hasn’t collided with other galaxies as much as some, although there do appear to be some strung out galaxies which have been ripped to pieces around it and of course there’s a big sector we can’t see behind the dark clouds obscuring the nucleus and we have little idea of what’s going on over there. However, what with M51 causing serious gravitational havoc in NGC 5915, there could be a lot of dangerous radiation pervading at least that side of the galaxy and the companion will also bring tidal forces to bear on the larger galaxy, and all of this is also likely to disturb planets within the galaxy because it will alter the orbits of the stars in it, causing them to approach each other more closely and disrupt the orbits of comets, leading to them hitting planets more often. But maybe not, who knows? The depths of spiral arms are also thought by some to be dangerous places for life because the times the Solar System has crossed them have been associated with mass extinctions, and this includes the Chicxulub Impact which wiped out the non-avian dinosaurs, which makes sense because of nearby stars disrupting the outer Oort comet cloud. The density of stars in M51 and its more tightly wound shape means the arms are more prominent than they are here, which means that whether or not NGC 5915 makes much difference to that, the observable fact of that density may have the same influence as the arms do on our own planet.

The Density Wave Theory, formulated by Lin and Shu, attempts to account for the grand design spiral form. It’s notable that the actual spiral arms rotate at a different rate than the stars do, and the arms represent a pattern of crowding of stars rather than stars permanently residing in the arms. They’ve been compared to traffic jams. This solves the “winding problem”, which is that the stars in the outer reaches of the arms ought to take longer to orbit the galaxy than those near the centre, leading to the arms appearing to wrap round and be obliterated. This does happen, but not to the extent it would be expected to. However, in a traffic jam the cars eventually accelerate out of the clump and were previously moving faster than it, but the jam itself moves a lot more slowly than the average speed of the stars within it, so the idea is that the arms represent waves of denser stars gradually moving around the galaxy. Saturn’s rings show a similar pattern.

Speaking of Saturn, the planet M51-ULS-1b is supposed to be about that size and to be orbiting at perhaps the same order of distance from its star as Saturn. Although it would be horribly inhospitable, because of the luminosity and heat of the B-type blue giant it orbits, it is a bit unusual for detected exoplanets in orbiting quite a long way from its star in a presumably fairly stable orbit. Further in and it would be a “Hot Jupiter”. Even taken alone, the star the planet orbits is quite hostile to planets per se, let alone life. However, because of the radiation from the dense member of the binary system, it still receives, or more likely received considering the age of the light relative to us, the level of radiation a hot Jupiter would.

X-ray astronomy may at first confound the imagination as to how it could ever be achieved. X-rays are not amenable to being focussed by lenses or mirrors because of their tendency to pass through solid objects easily, so how can there be an X-ray telescope? On the other hand, I’m guessing this also means being situated on the surface of a planet is less of a hindrance than it is for visible light and radio astronomy. Also, in theory X-rays have a much higher potential resolution than visible light, although I can’t imagine that’s easy to exploit. The smallest object it’s possible to image using visible light on the lunar surface from here would be 460 metres across, assuming no intervening atmosphere, but with X-rays this comes down to ten centimetres. However, focussing is problematic with X-rays and they have to be reflected off high iridium or ceramic mirrors at oblique angles, unlike mirrors for visible light. Materials transparent to X-rays don’t refract them, so a conventional lens of any substance can’t focus X-rays. The use of multiple mirrors to do this helps reduce the blurring of the image. Once focussed, X-rays can be detected using Geiger counter-like devices. There’s also a technique an astrophysicist once described to me involving two grids composed of substances relatively impermeable to X-rays which are brought in and out of phase, but I can’t remember how that worked. It was, however, remarkable in that a peace activist I knew heard the same explanation and regarded it as sinister and something which shouldn’t happen at all because of potential defence applications. Whereas I agree we should all be wary of such things, and aware of the links between belligerence and apparently pure scientific research, I don’t agree that everything space scientists do should be regarded with suspicion. To be honest, I think we’re relatively lucky that astronomical research as described in this post is still done at all, owing to people having apparently lost sight of anything remotely resembling the kind of things one might expect to go on in a rationally-organised society, such as the pursuit of pure curiosity, and I can understand that it might seem suspicious, but I still think astronomy is a very good thing.

Finding The Wrong Planets?

By ESA/Hubble, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=45642004

As of now, four thousand planets have been discovered orbiting other suns. Most of these have not been detected by pointing a telescope and seeing or photographing an actual, visible disc, or dot for that matter. A few may have been. For instance, the star Fomalhaut, also known as α Piscis Austrini, has a nearby dot which is not, however, orbiting in the plane of the star’s apparent discs of planetesimals, which has been seen through a telescope, but it appears that this is actually a cloud of débris, which probably explains why it’s visible from here. Therefore the main techniques use indirect evidence. There are two major methods used at the moment, although another was used in the past.

The most fruitful procedure uses the Doppler Effect. This works like this. A star has a spectrum with lines in it representing the elements which absorb particular wavelengths of light, so it looks like this:

This is the solar spectrum, although many of the lines have been omitted. Incidentally, this means that in theory if you look closely enough at a rainbow, you might expect to see gaps in the colour bands like this, but I imagine some kind of blurring effect takes place which makes it impossible. It’s generally known that if an emergency vehicle rushes past, the sound of the siren drops in pitch as it recedes from the listener. The same thing happens with light, which is why very distant objects in the Universe look redder than they would if they were stationary relative to Earth. The position of these bands is very precise because they’re defined by the energy levels of electrons in atoms, which in turn can be calculated using the principles of quantum mechanics. Therefore, quite minor movements in stars towards or away from us produce measurable alterations in the positions of these lines. And they do shift when planets are involved. Taking Jupiter as an example in our own solar system, it orbits around 5.2 AU (an AU, astronomical unit, is the mean distance of Earth from the Sun) from the Sun and has a mass about a thousandth of the Sun’s. Planets orbit in ellipses one of whose foci is ideally occupied by the centre of a star. However, this is not strictly accurate. In fact both the star and the planet orbit a common centre with the planet’s orbit much larger than the star’s. In the case of Jupiter and the Sun, this centre of gravity is a thousandth of the distance between the two, which is 150 000 kilometres from the centre of the Sun. Since the Sun’s radius is 696 000 kilometres, the furthest point of the photosphere from this point is around 850 000 kilometres away, and takes 11.86 years to move round an approximate circle of 5 340 000 kilometres at a velocity of only fourteen metres per second. This doesn’t sound fast enough to me to be detectable from anywhere or to have a notable influence on the absorption lines in the spectrum, and this is because Jupiter is far out and therefore doesn’t move the Sun around much. But suppose a planet had the same mass as Jupiter and only took a week to orbit the Sun. This would imply a semimajor axis (mean diameter of the orbit) of only twenty-two million kilometres and would lead to the Sun describing a 22 000 kilometre wide “orbit”, moving at around 400 kph. This is quite a bit faster, and shows how the easiest planets to detect by this method are those which are large and close to stars – “Hot Jupiters” as they’re known. This is known as the “radial velocity” method.

Another approach is to use transits, which are like solar eclipses in that they are when planets cross in front of stars. Taking the example of Jupiter again, if it were orbiting at a distance of eleven million kilometres from the Sun it would cross the disc once a week, taking somewhat under a day to do so, and would reduce the light seen from interstellar distances by about one percent, which is easily detectable. Many exoplanets have been picked up like this. The drawback is that the orbit of the planet round its star has to be aligned with ours. It’s no good for a planet whose orbit isn’t almost edge-on because that planet will never occlude its sun from our perspective. This is known as “transit photometry”. Like the radial velocity method, this is biassed towards large planets close to their stars, since they transit more often and cut out more light. One interesting side-effect of this method is that it can also be used to detect moons orbiting these planets, since like the stars in the radial velocity method, they will cause their primaries to describe small elliptical paths which will advance or delay their transit slightly, but because the planets detected are next to their stars they will usually lose the gravitational tug of war with them to retain any moons which might form or be captured.

There was an older technique which turned out to be an embarrassing failure. In the mid-twentieth century into the ’80s, attempts were made to detect the “wiggle” used by the radial velocity method when a star and planet were presumed to orbit at close to a right angle to Earth. Instead of moving in an almost straight line through space, a star and planet will always be slightly wavy. This method was thought to have detected planets in the 61 Cygni, Krüger 60 and 70 Ophiuchi systems, and orbiting Barnard’s Star, Lalande 21185 and ε Eridani. Strong doubts were cast upon the idea that variations that tiny could be detected from this distance, and in the end it turned out that they were caused by the lenses being taken out of telescopes and cleaned at regular intervals before being put back in in slightly different positions. Hence all of this lot went out of the window, which is a shame because the planets concerned were eminently “sensible”: they tended to look like the gas giants in our own solar system. It later turned out that Barnard’s Star does in fact seem to have a planet orbiting it at 0.4 AU which is several times the mass of Earth but probably rocky, if it exists, with a surface temperature likely to be around -170°C, assuming no atmosphere and so forth but still likely to be very cold. 70 Ophiuchi was actually the first system (it’s binary) to have the astrometric method used on it, in 1855, but it turned out that any body at the distance predicted would’ve been unstable. However, this has once been used successfully, to detect a planet seven times the mass of Jupiter orbiting Van Briesbock’s Star, a red dwarf nineteen light years away in the constellation Auriga.

Pulsars can also have planets. Pulsars are basically the remnants of collapsed massive stars not quite big enough to become black holes, and are effectively enormous atomic nuclei made entirely of neutrons inside and about ten kilometres across. They spin very fast, varying from about once every few seconds to about a thousand times a second, and if oriented in the right direction emit radiation from their poles which is detected as pulses from here, hence the name. Their spin is very precise, although it gradually slows and can vary due to star quakes. Although their surface gravity is ludicrously strong, this makes no difference to any of their possible planets because their mass is still almost the same as their parent stars, and they undergo the same perturbation as is used in the radial velocity method. This causes slight delays and advances to the pulses on a regular basis, and allows planets down to about the mass of Mars to be discovered. Unfortunately, because they’re outrageously radioactive and magnetic, there doesn’t seem to be any chance that any of them could support life as human beings can understand it – i.e. chemical and organic. The planet illustrated above has been named Poltergeist and orbits a pulsar designated PSR B1257+12, also known as Lich, along with three other planets. It’s about 2 300 light years away.

I mentioned previously that planets and moons can cause clumping and gaps in discs of matter orbiting their primaries. This happens with Jupiter and the asteroid belt. If you plot a bar chart of the periods of the asteroids in the belt, you get something like this:

If you then draw a diagram of these clumps, this emerges:

These are “rings” of asteroids whose time to orbit the Sun is almost in harmony with Jupiter’s in various ways, such as ⅔ or ½, named after prominent members of the clumps. Due to being regularly exposed to Jupiter’s gravity, they tend to be pulled away from their original orbits to ones which are slightly greater than that period. This is thought to be a factor in the formation of planets in our own Solar System. For instance, Earth orbits the Sun slightly less than a dozen times each Jovian year. Asteroids are far too sparse to be visible as rings from elsewhere in our stellar neighbourhood, but there are other star systems which still have the densely-strewn discs of planetesimals we once had while our own solar system was still formng, and in some of these gaps could be used to detect the locations of planets.

There are various other methods for detecting planets, but most of them suffer from the same problem. They tend to be much better at finding planets which are unlikely to be like ours. The transit and radial velocity methods tend to find the large close planets referred to as “Hot Jupiters”, and there do seem to be a lot of these. The astrometry method has so far only succeeded once and again needs a massive planet to work, as well as one which is quite far out. The pulsar timing method can easily detect Earth-sized planets but not ones which are remotely habitable because they’re close to powerful ionising radiation sources, and the detection of gaps in protoplanetary discs implies that planets are still forming in those systems. The difficulty is in identifying any Earth-like planets. However, the fact that there are a very large number of Hot Jupiters in itself makes the presence of such planets in their systems less likely. It’s possible that these planets started out at a considerable distance from their stars and fell inward. If this happened, they’d be likely to disrupt the orbits of any Earth-like planets further in, and their presence in an inner solar system from the start could also have a similar effect, so their very ubiquity could make the Universe a less friendly place for the likes of us.

However, this perception of the Universe as a place where Earth-like planets are rare could simply be an artifact of the methods used to detect planets elsewhere in the Galaxy. There are other ways to find planets but for several decades the only ones available were guaranteed only to locate those which were completely unsuitable for anything like us to live on. Consequently, for all we know there could be loads of planets like ours out there but we wouldn’t know about them.

I have to some extent caricatured this situation. In fact, although it’s true that none of the methods I mentioned are any good at discovering Earth-like planets, there are others which have been used more recently. For instance, there’s gravitational microlensing. The gravity of stars bends light passing from behind them, and this would be bent in a different way were there an accompanying local planet. This works better for planets viewed from further out from the centre of the Galaxy, because there are more stars near the centre than the edge, and are therefore more likely to be in the constellation of Sagittarius as that’s the direction of the centre of the Galaxy. One example is MOA-2007-BLG-192Lb:

This planet is three thousand light years away and is around 3.3 times our mass. This means that if it has the same density as Earth, it would have a surface gravity only 50% higher than ours and would also be a rocky planet like this one. However, since it orbits a brown dwarf, a star which is not large enough to have fusion reactions and is therefore quite cool, this planet is likely to be frozen solid and may even be considerably larger than that because it’s substantially made of ice. Even so, it’s a somewhat Earth-like planet which has been detected by this method.

To conclude then, there could still be plenty of Earth-like planets out there. Although the systems containing hot Jupiters may well be screwed, on the whole it’s currently diffcult to see planets like ours and the principle of mediocrity, which is that we should assume our situation is about average in the absence of further information as opposed to being special, suggests that we should presume that there are plenty of Earth-like planets until we can demonstrate otherwise. Therefore the currently rather hostile view we have of other solar systems is probably the result of how we look for planets, because we are selecting the “easy” cases rather than the ones which are most like our own situation. So far as we know, there are still likely to be many Earths out there and many solar systems like ours.