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How Minor Planets Are Discovered, From Blink Plates to Sky Surveys

A minor planet is found when a dot drifts against fixed stars. Here is the path from first image to confirmed orbit, official designation and credit.

ExplorationBy the duorolt.com editorial teamPublished Facts checked 9 min read

On February 18, 1930, Clyde Tombaugh sat at a blink comparator in Flagstaff, Arizona, and flipped between two glass photographic plates exposed six days apart. Most of the star images stayed put. One small dot jumped. That dot was Pluto, and the method that revealed it is still the core of how minor planets are found.

Today the plates are digital cameras and the flipping is done by software, but the logic has not changed. Stars hold fixed places from night to night. A minor planet, meaning an asteroid or another small body that orbits the Sun, drifts against them. Spotting the drift is only the first step. A find counts after other observations confirm it, an orbit is calculated, and the Minor Planet Center (MPC) accepts the data and issues an official designation. The MPC is the clearinghouse that receives position measurements, called astrometry, matches them against known objects and keeps the records.

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Blinking plates: how Tombaugh found Pluto

Tombaugh took long exposures at night with a camera that had a 33-centimeter (13-inch) lens. Each glass plate measured 36 by 43 centimeters (14 by 17 inches). He developed the plates and compared them during the day, according to the National Air and Space Museum.

The blink comparator is a special kind of microscope built by the Zeiss company. The operator turns a small dial that flips a mirror back and forth between the beams of two microscopes, so the two plates seem to alternate in the same spot. Anything fixed stays steady. Anything that moved appears to hop.

His plates from January 23 and 29, 1930 showed an object that moved too slowly to be an asteroid. If it was a planet, it lay beyond Neptune. Lowell Observatory announced the discovery on March 13, 1930, after further checks ruled out an asteroid or a comet.

The same idea still finds distant worlds. NASA says a Palomar Observatory survey team led by Mike Brown found Eris on January 5, 2005, in images taken on October 21, 2003. The object was already sitting in the data. It took until 2005 for someone to pick it out. That is one reason the list of dwarf planets and candidates is never final: every new survey can turn up another body that was waiting in the pictures.

How a modern survey finds a moving dot

A survey telescope points at a patch of sky, takes an exposure, moves on and returns to the same patch later. Software compares the images and flags anything that shifted. The numbers show how much sky this covers. According to the Catalina Sky Survey, its 1.5-meter (5-foot) telescope on Mt. Lemmon in Arizona covers about 1,000 square degrees of sky per night, down to a brightness of magnitude 21.5, using 30-second exposures. Magnitude is the astronomers' brightness scale, and larger numbers mean fainter objects. A 21.5 object is roughly 1.5 million times fainter than the faintest star most people can see in a dark sky. A smaller 0.7-meter (28-inch) Schmidt telescope on nearby Mt. Bigelow covers about 4,000 square degrees per night, down to magnitude 19.5.

One dot in one image proves nothing, because cosmic rays, satellites and camera flaws can all look like moving objects. The MPC recommends at least two observations of each object per night. The software links the dots into a tracklet, which is the short track of one moving object over a single night. A tracklet fits many possible orbits, so it takes more observations to narrow them down. For more on the instruments themselves, see the guide to the telescopes that find and study small bodies.

The newest survey machine is the NSF-DOE Vera C. Rubin Observatory in Chile. NSF reports that its camera takes about 1,000 images per night, one roughly every 40 seconds. Its ten-year Legacy Survey of Space and Time was announced as officially started on June 30, 2026. During early optimization surveys lasting about a month and a half, Rubin found over 11,000 previously unseen asteroids, including 33 near-Earth objects and about 380 trans-Neptunian objects, which are small bodies beyond Neptune.

From first sighting to confirmed orbit

The MPC describes a clear path from a raw detection to an official record.

Stage What happens Who does it
Detection A person or software finds a dot that moves; its position is measured against reference stars Observer
Report Positions go to the MPC in the ADES format or the older 80-column format Observer
Check The MPC compares them with known objects and sorts the rest MPC
Follow-up New possible near-Earth objects are posted so other telescopes can observe them Any capable observer
Provisional designation A code such as 2004 MN4 is assigned once the orbit is reliable MPC
Numbering A permanent number is given after enough well-observed oppositions MPC

In the check step, the MPC sorts what it receives into three bins. Objects that match a known minor planet are published as new observations of that object. Possible new near-Earth objects go to the NEO Confirmation Page, where other observers may follow them up. Tracklets that match nothing go to the Isolated Tracklet File, which the MPC checks periodically against newly computed orbits.

An opposition is the time of year when an object sits opposite the Sun in our sky, which is when it is best placed for observing. The MPC says main-belt asteroids typically need observations across about four oppositions before they get a permanent number. Near-Earth objects may qualify after two or three well-observed oppositions. That is why a provisional designation can sit on an object for years.

The flow of data is large. The MPC's December 2025 newsletter says it received 47.4 million observations during 2025, and that its database of published observations reached 521.5 million. In 2025, 6,331 objects appeared on the NEO Confirmation Page and 5,013 of them were designated as new objects (all figures as of December 29, 2025). The same newsletter notes that Rubin released its first images on June 23, 2025 and that the MPC took in the first Solar System data products from Rubin the same day. It also says the MPC does not plan to support the old 80-column format forever and urges observers to use ADES.

Who gets the credit

Credit is decided by a plain rule. The MPC says credit for a new provisional designation goes to the earliest identifiable observation at the time the designation is assigned. Earlier observations found later do not change that. To link an earlier observation to a new object, it must come from a different night, at least 12 hours apart, and be of good quality.

The rule has a sharp edge. If a colleague follows up your new object and reports it as new, credit goes to the colleague, even if you only had one night of data. A provisional designation also does not guarantee discovery credit in the long run. Only the discoverer, as the MPC defines that person, holds the privilege of suggesting a name once the object is numbered. That step is covered in the guide to how asteroids and comets get their names.

People without a telescope can still take part. NASA's page on the International Astronomical Search Collaboration says its volunteer teams have been credited with 120 numbered asteroids since 2006. The details are in the guide to citizen science for asteroids and comets. Anyone who tries real observing should remember that no telescope or binoculars should ever be pointed at the Sun without a solar filter made for that purpose. Survey work happens at night for a good reason.

Common mistakes and myths

"Seeing a new dot once is a discovery." One night of data gives a short arc, and many different orbits fit it. The find becomes solid only after more observations narrow the orbit down. Until then it may be reported as a candidate and followed up, or it may turn out to be a known object.

"The first person to see it names it." Naming is a separate step. It comes only after the object is numbered, and the privilege belongs to the discoverer as the MPC defines that person. Minor planet names cannot be bought.

"Discovery depends on huge telescopes alone." Big surveys find most objects, but the process needs people who measure positions, check candidates and follow up. Volunteer image-checking projects and amateur observers contribute real measurements.

"The orbit is known as soon as the object is found." A short tracklet gives only a rough orbit. Precise orbits come from months or years of observations, which is also why objects are sometimes lost and recovered later.

"Every new find is an asteroid." The MPC handles comets too. A fuzzy or tailed object may be reclassified after follow-up. The designation rules for comets differ and are explained in the guide to names.

Frequently asked questions

How many asteroids are known today?

As of November 11, 2026, JPL's Small-Body Database listed 1,580,112 asteroids, of which 895,910 were numbered. The total keeps rising because surveys add new objects every night.

Can an amateur find a minor planet from a backyard?

Yes, in principle. The MPC's guide for new observers says any telescope can be used, as long as you measure positions carefully, keep your clock accurate to UTC and use a modern star catalogue. In practice many beginners start by checking images supplied by a project instead of buying equipment.

What is the NEO Confirmation Page?

It is the MPC's public list of objects that look like possible near-Earth objects and need more observations. Observers around the world use it to decide where to point their telescopes next.

How long before a new object gets a permanent number?

Usually several years for main-belt asteroids, because the MPC wants observations from about four oppositions. Near-Earth objects can be numbered after two or three.

Sources