Interstellar Travel Opportunities.

As you may have heard, stars move. The only reason we cannot see this is because they are so far away, and it takes so long to make a difference. Currently, the closest star to us is Proxima Centauri, which is part of the Alpha Centauri grouping. It is 4.2 light years away, and if you think that is attractive for an interstellar voyage, just wait a bit. In 28,700 years it will be a whole light year closer. That is a clear saving in travelling time, especially if you do not travel close to light speed.

However, there have been closer encounters. Sholz’s star, which is a binary; a squib of a red dwarf plus a brown dwarf, came within 0.82 light years 78,000 years ago. Our stone age ancestors would probably have been unaware of it, because it is so dim that even when that close it was still a hundred times too dim to be seen by the naked eye. There is one possible exception to that: occasionally red dwarfs periodically emit extremely bright flares, so maybe they would see a star appear from nowhere, then gradually disappear. Such an event might go down in their stories, particularly if something dramatic happened. There is one further possible downside for our ancestors: although it is unclear whether such a squib of a star was big enough, it might have exerted a gravitational effect on the Oort cloud, thus generating a flux of comets coming inwards. That might have been the dramatic event.

That star was too small to do anything to disrupt our solar system, but it is possible that much closer encounters in other solar systems could cause all sorts of chaos, including stealing a planet, or having one stolen. They could certainly disrupt a solar system, and it is possible that some of the so-called star-burning giants were formed in the expected places and were dislodged inwards by such a star. That happens when the dislodged entity has a very elliptical orbit that takes it closer to the star where tidal effects with the star circularise it. That did not happen in our solar system. Of course, it does not take a passing star to do that; if the planets get too big and too close their gravity can do it.

It is possible that a modestly close encounter with a star did have an effect on the outer Kuiper Belt, where objects like Eris seem to be obvious Kuiper Belt Objects, but they are rather far out and have very elliptical orbits. It would be expected that would arise from one or more significant gravitational interactions.

The question then is, if a star passed closely should people take advantage and colonise the new system? Alternatively, would life forms there have the same idea if they were technically advanced? Since if you had the technology to do this, presumably you would also have the technology to know what was there. It is not as if you do not get warning. For example, if you are around in 1.4 million years, Gliese 710 will pass within 10,000 AU of the sun, well within the so-called Oort Cloud. Gliese 710 is about 60% the mass of the sun, which means its gravity could really stir up the comets in the Oort cloud, and our star will do exactly the same for the corresponding cloud of comets in their system. In a really close encounter it is not within the bounds of possibility that planetary bodies could be exchanged. If they were, the exchange would almost certainly lead to a very elliptical orbit, and probably at a great distance. You may have heard of the possibility of a “Planet 9” that is at a considerable distance but with an elliptical orbit has caused highly elliptical orbits in some trans Neptunian objects. Either the planet, if it exists at all, or the elliptical nature of the orbits of bodies like Sedna, could well have arisen from a previous close stellar encounter.

As far as I know, we have not detected planets around this star. That does not mean there are not any because if we do not lie on the equatorial plane of that star we would not see much from eclipsing observations (and remember Kepler only looks at a very small section of the sky, and Gliese 710 is not in the original area examined) and at that distance, any astronomer with our technology there would not see us. Which raises the question, if there were planets there, would we want to swap systems? If you accept the mechanism of how planets form in my ebook “Planetary Formation and Biogenesis”, and if the rates of accretion, after adjusting for stellar mass for both were the same, then any rocky planet in the habitable zone is likely to be the Mars equivalent. It would be much warmer and it may well be much bigger than our Mars, but it would not have plate tectonics because its composition would not permit eclogite to form, which is necessary for pull subduction. With that knowledge, would you go?

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Is There a Planet 9?

Before I start, I should remind everyone of the solar system yardstick: the unit of measurement called the Astronomical Unit, or AU, which is the distance from Earth to the Sun. I am also going to define a mass unit, the emu, which is the mass of the Earth, or Earth mass unit.

As you know, there are eight planets, with the furthest out being Neptune, which is 30 AU from the Sun. Now the odd thing is, Neptune is a giant of 17 emu, Uranus is only about 14.5 emu, so there is more to Neptune than Uranus, even though it is about 12 AU further out. So, the obvious question is, why do the planets stop at Neptune, and that question can be coupled with, “Do they?” The first person to be convinced there had to be at least one more was Percival Lowell, he of Martian canal fame, and he built himself a telescope and searched but failed to find it. The justification was that Neptune’s orbit appeared to be perturbed by something. That was quite reasonable as Neptune had been found by perturbations in Uranus’ orbit that were explained by Neptune. So the search was on. Lowell calculated the approximate position of the ninth planet, and using Lowell’s telescope, Clyde Tombaugh discovered what he thought was planet 9.  Oddly, this was announced on the anniversary of Lowell’s birthday, Lowell now being dead. As it happened, this was an accidental coincidence. Pluto is far too small to affect Neptune, and it turns out Neptune’s orbit did not have the errors everyone thought it did – another mistake. Further, Neptune, as with the other planets has an almost circular obit but Pluto’s is highly elliptical, spending some time inside Neptune’s orbit and sometimes as far away as 49 AU from the Sun. Pluto is not the only modest object out there: besides a lot of smaller objects there is Quaoar (about half Pluto’s size) and Eris (about Pluto’s size). There is also Sedna, (about 40% Pluto’s size) that has an elliptical orbit that varies the distance to the sun from 76 AU to 900 AU.

This raises a number of questions. Why did planets stop at 30 AU here? Why is there no planet between Uranus and Neptune? We know HR 8977 has four giants like ours, and the Neptune equivalent is about 68 AU from the star, and that Neptune-equivalent is about 6 times the mass of Jupiter. The “Grand Tack” mechanism explains our system by arguing that cores can only grow by major bodies accreting what are called planetesimals, which are bodies about the size of asteroids, and cores cannot grow further out than Saturn. In this mechanism, Neptune and Uranus formed near Saturn and were thrown outwards and lifted by throwing a mass of planetesimals inwards, the “throwing”: being due to gravitational interactions. To do this there had to be a sufficient mass of planetesimals, which gets back to the question, why did they stop at 30 AU?

One of the advocates for Planet 9 argued that Planet 9, which was supposed to have a highly elliptical orbit itself, caused the elliptical orbits of Sedna and some other objects. However, this has also been argued to be due to an accidental selection of a small number of objects, and there are others that don’t fit. One possible cause of an elliptical orbit could be a close encounter with another star. This does happen. In 1.4 million years Gliese 710, which is about half the mass of the Sun, will be about 10,000 AU from the Sun, and being that close, it could well perturb orbits of bodies like Sedna.

Is there any reason to believe a planet 9 could be there? As it happens, the exoplanets encylopaedia lists several at distances greater that 100 AU, and in some case several thousand AU. That we see them is because they are much larger than Jupiter, and they have either been in a good configuration for gravitational lensing or they are very young. If they are very young, the release of gravitational energy raises them to temperatures where they emit yellow-white light. When they get older, they will fade away and if there were such a planet in our system, by now it would have to be seen by reflected light. Since objects at such great distances move relatively slowly they might be seen but not recognized as planets, and, of course, studies that are looking for something else usually encompass a wide sky, which is not suitable for planet searching.For me, there is another reason why there might be such a planet. In my ebook, “Planetary Formation and Biogenesis” I outline a mechanism by which the giants form, which is similar to that of forming a snowball: if you press ices/snow together and it is suitably close to its melting point, it melt-fuses, so I predict the cores will form from ices known to be in space: Jupiter – water; Saturn – methanol/ammonia/water; Uranus – methane/argon; Neptune – carbon monoxide/nitrogen. If you assume Jupiter formed at the water ice temperature, the other giants are in the correct place to within an AU or so. However, there is one further ice not mentioned: neon. If it accreted a core then it would be somewhere greater than 100 AU.  I cannot be specific because the melting point of neon is so low that a number of other minor and ignorable effects are now significant, and cannot be ignored. So I am hoping there is such a planet there.