question: Why can we see Ultima Thule which is only 21 miles across, but can't seem to find a giant missing 9th planet?
comment: > why can we see ultimaThule. It should be noted that we can barely see ultima thule. Its was basically a couple pixels that popped up while searching for something to send new horizons after. We weren't specifically looking for it and if we where looking another month we probably wouldnt have found it and chose some other kuiper belt object to visit. Secondly Planet 9 is theorized to be **way** further away. It has a highly eliptical orbit, but even at its closet approach it is over 5 times further away than ultimathule and at its furthest the distance can be 30 times greater. Finally UltimaThule is just a rock that we found. Its not anything specific, and we would have probably sent a probe to whatever crossed the night sky when we where looking out that way. Planet 9 is a specific thing that we have to look for. Essentially requiring us to zoom in on a specific segment of the night sky and hope that we are looking at the right place at the right time. Its the difference between looking for a single needle in a haystack vs a haystack with 1000s of needles in them and any one would count.
passages: Section::::Detection attempts.
  
  Section::::Detection attempts.:Visibility and location.
   Other possible targets were Neptune trojans. The probe's trajectory to Pluto passed near Neptune's trailing Lagrange point (""), which may host hundreds of bodies in 1:1 resonance. In late 2013, "New Horizons" passed within of the high-inclination L5 Neptune trojan , which was identified shortly before by the New Horizons KBO Search survey team while searching for more distant objects for "New Horizons" to fly by after its 2015 Pluto encounter. At that range, would have been bright enough to be detectable by "New Horizons" LORRI instrument; however, the "New Horizons" team eventually decided that they would not target for observations because the preparations for the Pluto approach took precedence.
   Due to its extreme distance from the Sun, Planet Nine would reflect little sunlight, potentially evading telescope sightings. It is expected to have an apparent magnitude fainter than 22, making it at least 600 times fainter than Pluto. If Planet Nine exists and is close to perihelion, astronomers could identify it based on existing images. At aphelion, the largest telescopes would be required, but if the planet is currently located in between, many observatories could spot Planet Nine. Statistically, the planet is more likely to be close to its aphelion at a distance greater than 600 AU. This is because objects move more slowly when near their aphelion, in accordance with Kepler's second law. A 2019 study estimated that Planet Nine, if it exists, may be smaller and closer than originally thought. This would make the hypothetical planet brighter and easier to spot, with an apparent magnitude of  21–22. According to University of Michigan professor Fred Adams, within the next 10 to 15 years, Planet Nine will either be observable or enough data will have been gathered to rule out its existence.
   Because the planet is predicted to be visible in the Northern Hemisphere, the primary search is expected to be carried out using the Subaru Telescope, which has both an aperture large enough to see faint objects and a wide field of view to shorten the search. Two teams of astronomers—Batygin and Brown, as well as Trujillo and Sheppard—are undertaking this search together, and both teams expect the search to take up to five years. Brown and Batygin initially narrowed the search for Planet Nine down to roughly 2,000 square degrees of sky near Orion, a swath of space that Batygin thinks could be covered in about 20 nights by the Subaru Telescope. Subsequent refinements by Batygin and Brown have reduced the search space to 600–800 square degrees of sky. In December 2018, they spent 4 half–nights and 3 full nights observing with the Subaru Telescope.
   Section::::Detection attempts.:Ongoing searches.
   As of the end of 2018, no observation of Planet Nine had been announced. While sky surveys such as Wide-field Infrared Survey Explorer (WISE) and Pan-STARRS did not detect Planet Nine, they have not ruled out the existence of a Neptune-diameter object in the outer Solar System. The ability of these past sky surveys to detect Planet Nine were dependent on its location and characteristics. Further surveys of the remaining regions are ongoing using NEOWISE and the 8-meter Subaru Telescope. Unless Planet Nine is observed, its existence is purely conjectural. Several alternative theories have been proposed to explain the observed clustering of TNOs.
   This makes " a notably large body for how late it has been discovered, being the fifth largest plutino in the Solar System, after , Orcus, , and Ixion, and the largest discovered since Orcus in 2004. It is unknown exactly why no surveys had discovered it previously, as it is neither in a particularly dense region of the sky, nor far enough south that most northern hemisphere-based surveys would ignore it, being only 5–6° south of the celestial equator. It seems unlikely that any body as large as this one remains undetected in the plutino region.
  
  Section::::Physical characteristics.:Rotation period.
   On October 15, 2014, it was revealed that Hubble's search had uncovered three potential targets, temporarily designated PT1 ("potential target 1"), PT2 and PT3 by the "New Horizons" team. All are objects with estimated diameters in the range and were too small to be seen by ground telescopes. Each were at distances from the Sun of ranging from 43 to 44 AU, which would put the encounters in the 2018–2019 period. The initial estimated probabilities that these objects were reachable within "New Horizons" fuel budget are 100%, 7%, and 97%, respectively. All are members of the "cold" (low-inclination, low-eccentricity) classical Kuiper belt, and thus are very different from Pluto. PT1 (given the temporary designation "1110113Y" on the HST web site), the most favorably situated object, has a magnitude of 26.8, is in diameter, and was encountered in January 2019. A course change to reach it required about 35% of "New Horizons" available trajectory-adjustment fuel supply. A mission to PT3 was in some ways preferable, in that it is brighter and therefore probably larger than PT1, but the greater fuel requirements to reach it would have left less for maneuvering and unforeseen events. Once sufficient orbital information was provided, the Minor Planet Center gave provisional designations to the three target KBOs: (PT1), (PT2), and (PT3). By the fall of 2014, a possible fourth target, , had been eliminated by follow-up observations. PT2 was out of the running before the Pluto flyby. The spacecraft will also study almost 20 KBOs from afar.
   Sedna is however much closer than the supposed distance of the Hills cloud. The planetoid discovered at a distance of about 13 billion kilometers (90 AU) from the Sun, travels in an elliptical orbit of 11,400 years with a perihelion point of only 76 AU from the Sun during its closest approach (the next to occur in 2076), and travels out to 936 AU at its farthest point.
   Section::::Detection attempts.:Searches of existing data.
   Section::::Alternative hypotheses.:Shepherding by a massive disk.
   Section::::Physical characteristics.
  
  The size of depends on the assumed albedo (reflectivity); if it is a darker object then it would also have to be larger; a higher albedo would demand that it be smaller. The faint object has a visual magnitude of 24.64. Its brightness was comparable to one of Pluto's smaller moons. It is estimated by the discoverers to be in diameter, assuming an albedo of 0.15, Johnston's archive calculates a diameter of 386 km based on an albedo of 0.09.
  
  Section::::See also.
  
  BULLET::::- List of Solar System objects most distant from the Sun in 2018
   It would have been possible for the "New Horizons" spacecraft to investigate Neptune trojans discovered by 2014, when it passed through this region of space en route to Pluto. Some of the patches where the light from the Galactic Center is obscured by dust clouds are along "New Horizons"'s flight path, allowing detection of objects that the spacecraft could image. , the highest-inclination Neptune trojan known, was just bright enough for "New Horizons" to observe it in end-2013 at a distance of 1.2 AU. However, "New Horizons" may not have had sufficient downlink bandwidth, so it was eventually decided to give precedence to the preparations for the Pluto flyby.
   Other researchers have been conducting searches of existing data. David Gerdes, who helped develop the camera used in the Dark Energy Survey, claims that software designed to identify distant Solar System objects such as could find Planet Nine if it was imaged as part of that survey, which covered a quarter of the southern sky. Michael Medford and Danny Goldstein, graduate students at the University of California, Berkeley, are also examining archived data using a technique that combines images taken at different times. Using a supercomputer they will offset the images to account for the calculated motion of Planet Nine, allowing many faint images of a faint moving object to be combined to produce a brighter image. A search combining multiple images collected by WISE and NEOWISE data has also been conducted without detecting Planet Nine. This search covered regions of the sky away from the galactic plane at the "W1" wavelength (the 3.4 μm wavelength used by WISE) and is estimated to be able to detect a 10-Earth mass object out to 800–900 AU.
   As of December 2009, the smallest Kuiper belt object detected is 980 m across. It is too dim (magnitude 35) to be seen by "Hubble" directly, but it was detected by "Hubble"'s star tracking system when it occulted a star.
  
  Section::::Scattered objects.
   Section::::Constraints on additional planets.
  
  As of 2016 the following observations severely constrain the mass and distance of any possible additional Solar System planet:
  
  BULLET::::- An analysis of mid-infrared observations with the "WISE" telescope have ruled out the possibility of a Saturn-sized object (95 Earth masses) out to 10,000 AU, and a Jupiter-sized or larger object out to 26,000 AU. "WISE" has continued to take more data since then, and NASA has invited the public to help search this data for evidence of planets beyond these limits, via the Backyard Worlds: Planet 9 citizen science project.
   Astronomer Renu Malhotra remains agnostic about Planet Nine, but noted that she and her colleagues have found that the orbits of eTNOs seem tilted in a way that is difficult to otherwise explain. "The amount of warp we see is just crazy," she said. "To me, it's the most intriguing evidence for Planet Nine I've run across so far."
   Section::::Detection attempts.:Citizen science.
  
  The Zooniverse project, started in February 2017, is using archival data from the WISE spacecraft to search for Planet Nine. The project will also search for substellar objects like brown dwarfs in the neighborhood of the Solar System. 32,000 animations of four images each, which constitute 3 per cent of the WISE data, have been uploaded to the Backyard Worlds website. By looking for moving objects in the animations, citizen scientists might find Planet Nine.
   BULLET::::- "" (1999), computer game: A tenth planet called Dark Planet is not discovered for some time because it was obscured by the Kuiper belt.
   "New Horizons" made its first detection of on August 16, 2018, from a distance of . At that time, was visible at magnitude 20, against a crowded stellar background in the direction of the constellation Sagittarius.
  
  Section::::Encounter with.:Core phase.
    was discovered on 26 June 2014 using the Hubble Space Telescope during a preliminary survey to find a suitable Kuiper belt object for the "New Horizons" probe to fly by. Scientists were searching for an object in the Kuiper belt that the spacecraft could study after Pluto, and their next target had to be reachable on "New Horizons" remaining fuel. Using large ground-based telescopes on Earth, researchers began looking in 2011 for candidate objects and searched multiple times per year for several years. But objects that would work for "New Horizons" were just too distant and faint to be seen through Earth's atmosphere. was first spotted by Hubble on 26 June 2014, discovered by astronomer Marc Buie, a member of the "New Horizons" team.
   The search of databases of stellar objects by Batygin and Brown has already excluded much of the sky along Planet Nine's predicted orbit. The remaining regions include the direction of its aphelion, where it would be too faint to be spotted by these surveys, and near the plane of the Milky Way, where it would be difficult to distinguish from the numerous stars. This search included the archival data from the Catalina Sky Survey to magnitude c. 19, Pan-STARRS to magnitude 21.5, and infrared data from the Wide-field Infrared Survey Explorer (WISE) satellite. They have more recently also searched the first year data release from the Zwicky Transient Facility without identifying Planet Nine.
   Distances between the planets in the Solar System are often measured in astronomical units (AU), defined as the average distance between the Sun and Earth, some . Venus, the closest other planet to Earth is (at closest approach) 0.28 AU away. Neptune, the farthest planet from the Sun, is 29.8 AU away. As of January 2018, Voyager 1, the farthest man-made object from Earth, is 141.5 AU away.
  
  The closest known star, Proxima Centauri, is approximately away, or over 9,000 times farther away than Neptune.
   "New Horizons" passed within of Pluto, with this closest approach on July 14, 2015, at 11:50 UTC. "New Horizons" had a relative velocity of at its closest approach, and came as close as to Charon. Starting 3.2 days before the closest approach, long-range imaging included the mapping of Pluto and Charon to resolution. This is half the rotation period of the Pluto–Charon system and allowed imaging of all sides of both bodies. Close range imaging was repeated twice per day in order to search for surface changes caused by localized snow fall or surface cryovolcanism. Because of Pluto's tilt, a portion of the northern hemisphere would be in shadow at all times. During the flyby, engineers expected LORRI to be able to obtain select images with resolution as high as if closest distance were around 12,500 km, and MVIC was expected to obtain four-color global dayside maps at resolution. LORRI and MVIC attempted to overlap their respective coverage areas to form stereo pairs. LEISA obtained hyperspectral near-infrared maps at globally and for selected areas.
   From Deimos, Mars appears 1,000 times larger and 400 times brighter than the full Moon as seen from Earth, and appears as a sphere of angular diameter roughly 16.5°—taking up an eleventh of the width of a celestial hemisphere.
  
  Section::::The Asteroid Belt.
  
  The asteroid belt is sparsely populated and most asteroids are very small, so that an observer situated on one asteroid would be unlikely to be able to see another without the aid of a telescope. Occasional "close approaches" do occur, but these are spread out over aeons.
  
presuppositions: We easily saw Ultima Thule even though it is small. We can see Ultima Thule.
corrections: We barely saw Ultima Thule and almost passed over it. We can barely see Ultima Thule.
raw_presuppositions: We easily saw Ultima Thule even though it is small. We can see Ultima Thule.
raw_corrections: We barely saw Ultima Thule and almost passed over it. We can barely see Ultima Thule.