Imagine an astronaut lost on another world when their communication systems fail. In most cases, they won’t be able to use a compass for directions, since that depends on magnetic fields. In this event, the traveler may need to find their way home by the stars, but how easy will that be? In particular, will they have a guide like Polaris, a star that appears to barely move, always marking the way north or south, to keep them orientated?
The answer, you will not be surprised to learn, depends on the planet in question.
The presence of a pole star in the northern hemisphere, and its absence in the south, is a historical coincidence. As we recently explained, the Earth’s poles draw a great circle in the sky over a 24,000-year cycle, known as precession. This means that the star we call Polaris has been the closest visible marker of the celestial North Pole for about less than 4,000 years, and wasn’t all that close for much of that time. Consequently, it’s not surprising few other planets in our Solar System are as well supplied.
Mercury
Being the planet closest to the Sun, Mercury is the one most under our star’s spell, including having a closely aligned axis of rotation. Like the Sun, its north pole points towards Draco, a constellation notably lacking in bright stars, roughly equidistant between Xi and Delta Draconis. Delta Draconis was found to be Ceres’ Pole Star, by the Dawn mission, but it’s not easy to distinguish in the sky. Mercury’s southern hemisphere does better. The nearest star to its south celestial pole is Alpha Pictoris, which is similar in brightness to the Draco pair, but at least has the decency to be almost perfectly placed.
Venus
Any lost astronaut on Venus is thoroughly done for. Even if their suit is capable of repelling the blast-furnace temperatures, they won’t be able to look to the skies for help. The clouds that prevent us from seeing the Venusian surface other than with radar would just as effectively prevent anyone on that surface from looking up.
The International Astronomical Union had to settle a debate as to which pole on Venus should be considered north, and which south, given its unusual spin. In this case, it matters little, as neither pole is blessed with much in the way of pole stars, even if the clouds parted.
Mars
Of course, most people dreaming of striding across another world imagine the red soil of Mars beneath their spacesuit’s toes. The Martian poles are tilted at quite a similar angle to their planet’s orbital plane to Earth’s (25.2 versus 23.4).
However, as our own precession shows, a planet can maintain a constant tilt while still pointing at different spots in the sky. (Earth’s axial tilt also changes, but over a much narrower range and more slowly).
Mars’ north pole points to a location in Cygnus almost 30 degrees away from Polaris, with only barely visible stars nearby. Sadly, the closest easily visible stars are about 9 degrees away, not great for guidance. On the plus side, one of those stars is Deneb, the 19th brightest star and therefore very easy to find.
The Martian South Celestial Pole points towards the constellation Vela. However, amateur astronomers know the area better as the “false cross”, a larger and fainter neighbor of the more famous Southern Cross, composed of stars from Vela and Carina. A respectably bright member of the False Cross quartet, Kappa Velorum, is just 3 degrees away, close enough to serve as a pole star. It’s fainter than Polaris, and more distant from the relevant celestial pole, but still one of the best pole stars of any planet.
The Gas Giants
You might expect we’d need to abandon our hypothetical astronaut here, since no one will walk on these worlds’ gaseous layers. However, the question of whether the giant planets have pole stars could actually be quite relevant.
According to blogger Grant Hutchison, the gas giants’ major satellites have axes of rotation that are closely aligned to those of their planets. We know this isn’t a universal feature of satellites, because our own Moon’s axis is quite misaligned with Earth’s, but it seems the giant planets have done a better job of keeping their offspring in line. Oddly, our Moon’s axis is much more closely aligned with that of Jupiter and Venus than our own, which feels almost treacherous on its part.
Such alignments are never perfect, so in some cases a planet may have a pole star that doesn’t really work for at least some of its moons. Nevertheless, as a starting point, knowing a giant planet’s pole star (or lack thereof) is a very big hint for the surrounding worlds we may one day explore.
Jupiter’s axis is almost at right angles to the plane of its orbit, just 3.1° off. Since there isn’t much difference between Jupiter’s orbital plane and Earth’s, Jupiter’s axis is not far off perpendicular to Earth’s, very different from our own 23.4°. Among other things, that means the axis is not that far off that of the Sun, and similarly places the north celestial pole in Draco, although not quite as close as Mercury’s. Once again there are several visible stars in the rough vicinity of the pole, but none of them particularly close or bright. However, as with Venus, the Large Magellanic Cloud serves as a good rough guide to Jupiter’s South Celestial Pole.
Saturn’s axis is the most closely aligned with Earth’s of any planet and we share a north pole star. However, Polaris is considerably further from Saturn’s north celestial pole than our own, so Saturn, and its moons, are not nearly so well served. Saturn’s south celestial pole is very close to Delta Octans, but that’s such a disappointingly faint star it’s barely useful.
Uranus famously lies on its side, and its celestial poles are nowhere near those of any other planet. Its north pole points almost straight at Eta Ophiuchi, also known as Sabik, which is not quite as bright as Polaris, but not a bad substitute. The south pole doesn’t have a nearby star, but is a little closer to Aldebaran than Betelgeuse. Finding it precisely is therefore very difficult, but the two bright red giants make it easy to get to the general vicinity.
Neptune’s north celestial pole is a little closer to Deneb than Mars’ is, but it also has the fainter Delta Cygni respectably close. Its south pole has Gamma Velorum, which is a little brighter than Polaris, close enough to use.
Other Worlds
When it comes to planets beyond the Solar System there is a simple answer to our question: we don’t know. We can work out how the stars would appear on planets surrounding nearby stars, but we have no idea of the orientation of their planets’ poles even in the cases where we know the planets exist.
