The Week in Space and Physics: The Dwarf Planets
On a new view of the outer dwarf planets, the search for planet X, a star with a scar and the revival of SLIM
For much of its time the James Webb telescope is focused on the distant universe. It has spied out exoplanets around faraway stars, scanned gas clouds thousands of light years away and even peered across billions of years of space and time into the early cosmos.
Sometimes, however, scientists direct it towards a closer target. The telescope has created beautiful images of Saturn and Jupiter, tracked temperatures on Mars and captured rings and moons circling around both Uranus and Neptune. These observations are not just for fun: the telescope has an exceptional ability to pick out the gases floating around other worlds, and so to track activity taking place across the Solar System.
Dotted around the edge of our solar system is a group of dwarf planets collectively known as trans-Neptunian objects. Some, like Pluto and Triton - a former member of the club that has since become a moon of Neptune - are well known, and relatively well studied. Others, like Eris, Makemake and Sedna, are more mysterious, appearing as little more than a dot of light in a telescope.
For some time, however, there have been clues that these worlds are more than just frozen lumps of ice. They have bright surfaces, which suggests that something must be regularly reshaping them. Triton, when Voyager 2 flew past in 1989, turned out to host active geysers and possibly a liquid ocean under a frozen shell. Pluto, seen by New Horizons in 2015, has a system of icy volcanoes and a youthful surface.
With no other probes heading to the outer solar system, prospects for a close up examination of other trans-Neptunians like Eris or Makemake are dim. As an alternative, astronomers have turned to the James Webb telescope. It has recently examined several of these worlds and mapped out the details of the gases and chemicals found on their surfaces.
As other telescopes have done, the James Webb saw signs of frozen methane on the surfaces of Eris and Makemake. Its extra power, however, was able to measure details of the hydrogen contained within that methane. More specifically, it looked at the ratio of normal hydrogen atoms - with no neutrons - to those of deuterium - hydrogen atoms with a single neutron.
If Eris and Makemake were simple lumps of ice, with no internal activity to speak of, we would expect that ratio to be the same as seen in other icy objects in the outer solar system - most of which are comets. Yet the data from the James Webb shows a different ratio. The conclusion, scientists argued in a recent paper, is that both Eris and Makemake are more complex worlds, featuring systems of geysers and icy volcanoes.
More work, they say, is needed to confirm this. The James Webb can help, especially by calculating similar ratios for more comets and trans-Neptunian objects. But to really find answers about these worlds we should send a probe to take a closer look. That possibility, however, is a long way off: even if we had a rocket ready to launch tomorrow, it would take at least two decades to reach either Eris or Makemake.
The Search for Planet X
The first trans-Neptunian object, Pluto, was spotted in 1930. Sixty-two years passed before we found another, but ever since the floodgates have been opened. At least two thousand of them are now known, though most are no more than a few dozen miles across.
All known trans-Neptunian objects are small. Pluto is still the largest - but with a diameter of less than two thousand miles, it is significantly smaller than our Moon (and even, were you to place one on top of the other, smaller than the United States of America).
Some still wonder, however, if another large planet is lurking out there. They point to patterns in the orbits of the trans-Neptunians as evidence for this, alleging that ‘Planet X’ may be pulling them into alignment. Whether these patterns really exist is still unclear, as is the actual cause if they do.
Still, discovering another planet would be one way to get immortalised in the history books, and so many astronomers have taken to looking for one. One previous study by Michael Brown and Konstantin Batygin at Caltech attempted to nail down the orbit of Planet X, based on the alleged patterns seen in the trans-Neptunians.
The problem, however, is that even if the orbit can be deduced, the exact location of the planet cannot. Astronomers would need to search a vast swath of the sky to find it, and that would take a lot of time. Fortunately we do have regular and extensive surveys of the night sky. Brown and Batygin have now published results of an analysis of Pan-STARRS data, one such survey.
They did not find the planet. They did, however, succeed in ruling out almost eighty percent of the possible places it could be hiding, at least based on their earlier predictions. The remaining twenty percent should be covered by other telescopes within the next few years. If Planet X is out there, they confidently declare, it will be found before long.
A Star with a Scar
Sixty light years from Earth is a star with a strange metallic scar, according to a recent study. The star in question is a white dwarf, the kind of small, faint star that our Sun will eventually become in billions of years time. The scar, which stretches across one side of the dwarf, is probably the resting place of a shattered planet.
Earlier studies of this star had picked out signs of metals - especially calcium, magnesium and iron - on its surface. Such signs are not uncommon in white dwarfs. As a sunlike star begins to die, it first expands into a red giant. Any nearby planets will vaporise, evaporating into a stream of metallic particles. As the red giant shrinks into a white dwarf, those particles will fall onto the star, ‘polluting’ its surface with metal.
Oddly, however, two earlier studies of this star had reached different conclusions. One found clear signs of metal, marking it out as a star that had eaten its planets. Another found no signs of metal - and, therefore, no trace of past planets.
To solve this disparity, astronomers took a longer look at the star last year. They found the signs of metals came and went as the star rotated, showing that the metal was present on only one side of it. That, they say, points to a metallic scar smeared across its surface. Most likely the particles were caught in the star’s magnetic field as they fell inwards, and thus guided to a specific part of its surface.
Such scars might be common in white dwarfs. Yet past studies have typically used just a single image of these stars to judge their metal content. If the scar happened to be facing away from us at the time of the image then we would miss it. That means, too, that far more white dwarfs have consumed their planets than previously thought.
SLIM Revives
Japan’s SLIM moon lander reawoke last week after unexpectedly surviving the harsh lunar night. The probe, which suffered from power issues after tipping over during its landing, managed to send back data and images of a nearby crater.
Since our moon spins only once every twenty-eight days, its nights are long - fourteen days in length each. Even worse, the Moon has no atmosphere to trap heat, and so temperatures fall to extreme lows in the dead of night. SLIM likely endured conditions hundreds of degrees below freezing.
Since the spacecraft was not designed to survive such temperatures, mission planners originally expected the mission to close as night fell. Fortunately, however, SLIM did better than expected. Operators managed to get a few days of work, including some new photos, from the probe after it rewoke. Unfortunately the harsh lunar conditions soon returned: as night returned to the the surface, SLIM’s operators once again put it to sleep.