The science of rare planetary alignments

The alignment of both the Sun and the Earth with another planet in the Solar System is a rare event, which we are seldom able to observe in a lifetime. The Sun-Venus-Earth alignment for example only takes place once every 105.5 or 121.5 years. Similarly, the next Sun-Earth-Mars alignment will only occur in 2084. But on 5 January 2014, we were lucky enough to witness one such rare event: the alignment of the Sun, Earth, and Jupiter. Much to our surprise, we saw a new physical effect never observed before.

During these alignments, the planet farthest away from the centre of our Solar System sees the other planet crossing, or transiting, in front of the Sun. During the Sun-Earth-Jupiter transit for example, observers on Jupiter would see the Earth transiting in front of the Sun. No such transit would be visible from the Earth, however we have worked out an ingenious technique to “observe” these transits using the outer planet in the alignment as a reflecting mirror. Instead of directly observing the transit, we can observe the light being reflected by the outer planet as the transit takes place to study the effect the transit has on the light coming from the Sun.

After having first applied this technique to the transit of Venus in 2012, during which light reflected by the Moon was used to study Venus, we used this method to observe the Earth during the Sun-Earth-Jupiter alignment. It was like following the passage of the Earth in front the Sun while comfortably sitting on Jupiter. Or more precisely, on one of its moons – Ganymede or Europa – since Jupiter itself, due to its high rotational speed and turbulent atmosphere, is by no means an ideal mirror.

The scientific aim of the observations was to study the Earth’s atmosphere as sunlight passed through it, and to measure the small drift in the positions of the spectral emission lines due to the occultation of part of the solar disc. Methods like these are also being used to study properties of the multitude of exoplanets currently being discovered (e.g. by NASA’s Kepler mission), in order to infer atmospheric composition and orbital parameters.

The event was followed from the Galileo telescope at La Palma, Canary Islands, and from the European Southern Observatory’s 3.6m telescope at La Silla in Chile. These were the only sites in the world able to observe the transit with the necessary high precision spectrographs. But instead of the expected decrease in the solar brightness due to the eclipse, we actually saw an increase. At the beginning we thought we had made a mistake during one of the observations, or that something in the instrumentation had not worked properly. We double-checked all the possible causes but found nothing unusual or wrong. Finally, after almost a year, we realised what had happened, and step by step we were able to interpret what had seen: a totally new physical effect never measured before.

This is a peculiar effect that occurs because Jupiter’s moons Europa and Ganymede have no atmosphere – the light from the source, i.e. the Sun, comes from directly behind the observer on Earth. You may remember the light surrounding the head of the shadow of astronauts on the Moon.

The observed increase in brightness occurs only when the alignment is perfect. As it moved through the disk of the Sun, the Earth’s atmosphere was acting as a lens and increasing the intensity of the light  from the areas of the Sun around its projected image. The result on the spectral lines was to move them exactly the opposite of an eclipse, and by far stronger. Our model explains the observations in every small detail.

The next alignment between the Sun, Earth, and Jupiter will occur pretty soon in 2026. After that, we will have to wait until 2109. We really hope to have a second chance to follow this new transit and confirm our theories with the new Extremely Large Telescope under construction in the Atacama desert in Chile.

Featured image: The Solar System by Harman Smith and Laura Generosa, Public Domain via Wikimedia Commons.