Sen—2014 was the year of the comet, but 2015 is certainly turning into the year of the dwarf planet. NASA's Dawn mission entered orbit around Ceres in the asteroid belt on March 6th, meanwhile in the outer Solar System at the edge of the Kuiper belt, NASA's New Horizons is on course for a mid-July close fly-by of the Pluto system.
Ceres and Pluto are deemed dwarf planets with the classification scheme adopted by the International Astronomical Union (IAU) in 2006. Dwarf planets are large enough that gravity can make them round, but they are not massive enough to clear the neighborhood around their orbits. These objects are part of the remnants left over after planet formation in the Solar System, but this class of bodies stands out as distinct from the smaller planetesimals and the classical planets.
The dwarf planets are dynamic and active worlds intriguing in their own right. Just last year, the discovery of water vapor being actively emitted from regions on Ceres' surface was announced. Meanwhile Pluto is known to have an atmosphere and weather where the volatile ices on its surface are evaporating from the Sun-lit pole and are transported and freeze out on to its winter hemisphere. With New Horizons and Dawn, we have a chance to study these bodies in such never-before-seen detail. Dawn is already beaming back images of Ceres' surface with better resolution than what can be achieved with the Hubble Space Telescope (HST). New Horizons has officially started imaging Pluto and its large moon Charon as part of the early stages of its science campaign, but we'll have to wait to around May for images of the surface to be sharper than what HST's keen eyes have been able to provide.
So with all this talk about Pluto and Ceres and with more to come over the next few months, I thought I'd touch on our prospects of finding more dwarf planets in the Solar System. How do we find these bodies?
The general technique used to finding Solar System objects hasn't changed much over the decades though the tools have improved. You take an image of the sky with your telescope. Then you come back to the same spot a few hours to days later and take another image. From night to night, you can consider the background stars in essence stationary objects, and anything that you see changing position from one image to the next is a moving Solar System body.
In the days of Clyde Tombaugh's search for Planet X, telescopes were imaging to photometric plates, and Tombaugh used a device known as a blink comparator to switch views between the two observations of a given field. By 'blinking' the two observations, your eye can easily spot the moving blip, and on February 18, 1930, on one of those blinked plates Tombaugh discovered Pluto. Since then the technique has evolved some. Now telescopes are equipped with digital CCD cameras that continue to increase in field-of-view. Computerized searches of the images are now performed rather than scanning the entire image by eye, but the resultant candidates are still reviewed by a human blinking the digital subframes. During my graduate thesis while searching for distant small Solar System objects, I reviewed in total ~40,000 blinking snapshots of candidate moving objects over a 3 year period.
The asteroid belt is much closer than the Kuiper belt, and current telescopes would have found another object like Ceres if it existed between Mars and Jupiter. It would be extremely hard to have such a large body missed by current asteroid surveys. So Ceres will remain the only dwarf planet in the asteroid belt for the foreseeable future, but there are better chances to find a dwarf planet beyond Neptune. One reason is due to composition. Further out from the Sun, the ice fraction goes up and it's easier to melt ice into a sphere than it is rock. So in the outer Solar System smaller objects are round compared to their counterparts in the asteroid belt. In addition, there's more places in the outer Solar System to hide a dwarf planet undetected up to present day. The further out beyond Neptune, the fainter they are and the bigger the telescope required to detect them. To give some sense of scale, it took 62 years for the first Kuiper belt object after Pluto to be discovered.
With robotized mid-scale telescopes and the switch from photographic plates to CCDs, a slew of relatively bright Pluto-sized objects (18-21st magnitude) residing in the Kuiper belt were discovered. The majority of these bodies were discovered by Mike Brown and collaborators using the 48 inch Palomar Oschin Schmidt telescope including Eris, the Pluto-sized object whose discovery spurred on Pluto's planetary demotion. Each one of these objects (including Makemake, Pluto, Eris, Haumea, Sedna, Quaoar, 2007 OR10, and Orcus) is bright enough for detailed study, and each has a unique story to tell. For example, Haumea has been involved in a mantle shattering collision that has flung the icy fragments from its interior out into the Kuiper belt, and Makemake’s volatile-rich surface is a photochemical laboratory where methane is churned into ethane, propane, and other higher order hydrocarbons.
Since the discoveries of Eris and friends in the 2000s, the dwarf planet discoveries have dwindled. It's not been for lack of trying. Surveys have mainly come up empty. Previous work I've been involved in has suggested that we're at the end of finding large objects in the Kuiper belt brighter than ~21st magnitude with the expected number of bright objects like Pluto and Eris matching close to what already's been discovered. Recent work by Mike Brown and collaborators repurposing archival Near Earth Asteroid survey data to search for very bright Pluto-sized objects has turned up empty. No new large dwarf planets brighter than 20th magntude were discovered. Based on their estimates there's only a 32% chance of there existing a very bright large Kuiper belt object that hasn't been discovered yet. This evidence points to the inventory of bright dwarf planets (brighter than 20th magnitude) in the outer Solar System being complete, but there could be many fainter Pluto-sized bodies orbiting further out in the Kuiper belt and inner Oort cloud still waiting to be found.
We'll probably have to wait from the 8-m Large Synpotic Sky Survey Telescope (LSST) to turn on in the 2020s to find the next batch of dwarf planets, but just last year, there was the announcment of a faint 23rd magnitude dwarf-planet sized body (2012 VP113—a new inner Oort cloud object) at ~80 AU discovered using the Dark Energy Camera (DECam) on the 4-m Blanco telescope in Chile.