How Do Astronomers Find and Measure Tiny Worlds Beyond Neptune?

What Is Out Beyond Neptune, and Why Did the Next Discovery Take 62 Years?

Beyond Neptune lies a wide ring of leftover icy bodies called the Kuiper belt (the name is pronounced roughly “KY-per”). NASA describes it as starting at about 30 astronomical units from the Sun and its main region ending around 50. One astronomical unit, or au, is the distance from the Earth to the Sun [4]. Pluto, found in 1930, turned out to be the first Kuiper belt object ever discovered [4], though nobody knew that at the time.

The next one was announced on August 30, 1992. It was found by David Jewitt and Jane Luu using a telescope on Mauna Kea in Hawaii, and it was given the label 1992 QB1. It was later named (15760) Albion [2]. It is described as the first object found beyond Neptune after Pluto and its moon Charon [2]. Jewitt, then at the Massachusetts Institute of Technology, had been puzzled by the apparent emptiness of the outer Solar System, and Luu was his graduate student [1]. Between 1930 and 1992 there are 62 years (that is my subtraction, not a number from the sources).

The sources do not give one simple reason for the 62-year gap. Here is what their facts suggest. Two things made these objects hard to find: they are very faint, and they move slowly. The next two sections take those one at a time.

Why Are Objects Beyond Neptune So Faint?

They make no light. Like the Moon, they only bounce back sunlight, and the light has to make a round trip: sunlight travels from the Sun to the object, then the reflected light travels from the object to us. Light thins out as it spreads, so it gets weaker with distance. Sunlight at twice the distance is one quarter as strong. That weakening happens on the way out and again on the way back.

Put the two trips together and a rough rule appears. For objects of the same size and reflectiveness, the reflected light we receive falls with the fourth power of their distance from the Sun. This is my own simple estimate, not a figure from the sources, and it is only a rough one. It treats the Earth as sitting close to the Sun compared with the object, which is fine for something as far away as 40 au. By this rule, an object at Jupiter’s distance (about 5 au) would be 625 times fainter than the same object at 1 au. An object at 40 au would be about 2.5 million times fainter, since 40 × 40 × 40 × 40 is 2,560,000.

Because of that, the astronomers who found 1992 QB1 were not looking for something you could see with a backyard telescope. The Wikipedia page for Albion lists its brightness as magnitude 23.3, but the page does not say when that was measured [2]. Magnitude is the astronomers’ brightness scale, and bigger numbers mean fainter. The faintest stars you can see with your eyes under a dark sky are around magnitude 6, so an object of magnitude 23 is millions of times fainter.

How Do You Spot One Dot That Moves Among Millions of Stars?

Faintness is only half the problem, because a faint dot is easy to miss in a crowded sky. The trick is movement. The stars stay put relative to each other, but a Solar System object drifts against them. So you photograph the same patch of sky on different nights and look for a dot that has shifted. Clyde Tombaugh used this method in the 1930 search that found Pluto, and Jewitt and Luu began with the same blink-comparator approach [1]. A blink comparator is a viewer that flicks back and forth between two photographs so that anything that moved seems to jump.

Checking by eye was slow. According to the Wikipedia page on the Kuiper belt, examining a single pair of photographic plates initially took about eight hours [1]. The speed-up came from a CCD, the kind of electronic light sensor now found in digital cameras. The same page says CCDs retained about 90% of the light that hit them, compared with about 10% for photographs, so they could record fainter objects [1]. Pictures could also be compared on a computer screen instead of on glass plates. The whole search took about five years before 1992 QB1 turned up [1].

Once one was found, the belt began to fill in. NASA says more than 2,000 trans-Neptunian objects have been cataloged [4], and it estimates hundreds of thousands of objects larger than 100 km across [4].

Why Can’t Brightness Tell You How Big a Distant Object Is?

Once you have a dot, the natural next question is how big it is. Brightness alone does not say. A big dark object and a small shiny one can look equally bright. Even the first catch shows the problem: the Albion page gives a diameter of about 108 to 167 km, and the range comes from different guesses about how reflective its surface is [2]. A guess for reflectiveness gives a guess for size.

A better method is to use a star as a ruler. When a distant object passes exactly in front of a star, the star winks out for a short time. This event is called an occultation. The object’s shadow sweeps across the Earth, and the width of that shadow is close to the width of the object. If you stand inside the path of the shadow with a telescope, you can time how long the star vanishes. Each telescope measures how long the star disappears. From that time, astronomers work out one line across the object, called a chord. Enough chords give an outline.

How Did 24 Telescopes in Argentina Catch the Shadow of a 30-Kilometer World?

The test case was a small object named Arrokoth. It was found on June 26, 2014, by Marc Buie and the New Horizons team using the Hubble Space Telescope, during a search for a target the New Horizons spacecraft could visit after Pluto [5]. Then the team had to find out its shape before the spacecraft arrived.

In 2017 they tried three stellar occultations [3]. The first, on June 3, failed. Teams in Argentina and South Africa found no shadow, and later Hubble data showed that the telescopes had been placed in the wrong locations [5]. On July 10 a telescope on the aircraft SOFIA flew over the Pacific. At first, the central shadow seemed to have been missed. In January 2018, the team recognized a brief dip in the star’s light [5]. On July 17 the team placed 24 mobile telescopes in a line across southern Argentina, like a fence across the path of the shadow. Five chords clearly showed a complicated shape consistent with a contact binary (two lumps stuck together), with rough dimensions of 20 by 30 km and a visible albedo (reflectiveness) of 10% [3].

On January 1, 2019, New Horizons flew past at a distance of about 3,500 km, and NASA calls Arrokoth the most distant object ever explored up close [6]. The close-up images showed two joined lobes, with the whole object about 35 km long [6]. The occultation outline had been about right. The best-fit shape model measures about 34.5 by 19.8 by 13.8 km [5]. A star blinking out on Earth had told astronomers the shape before the spacecraft ever got close.

Can Astronomers Find Kuiper Belt Objects Too Small to See?

Surprisingly, yes, though only as a shadow. That page estimates, under its own assumptions, that objects smaller than 1 km in radius would be around magnitude 35, too faint for even Hubble to photograph directly [1]. But a tiny object can still pass in front of a star. In 2009, Hilke Schlichting and her colleagues reported a dip lasting about 0.3 seconds in old Hubble guiding data from March 2007. It pointed to an object with a radius of about 520 meters [1]. A second event, with a radius of about 530 meters, was reported in 2012 [1].

From these two events, the Wikipedia page says the authors found fewer small objects than you would get by extending the numbers for larger ones downward [1]. Two events are very few, so I would not treat that as settled. I did not read the original Schlichting papers.

How Can You Try an Occultation Yourself?

• Turn on one small lamp at the far end of a room (not the Sun, and nothing very bright). Hold a pencil at arm’s length in front of one eye so that it hides the lamp. Now keep your body still and move only your head sideways. How far can your head move before the lamp reappears? That range is your own “shadow path.” Is it wider than the pencil, or about the same?
• Think about the fence of 24 telescopes. If the shadow is only 20 to 30 km wide, why did being a few kilometers off on June 3 matter so much?
• Read the abstract of Buie and colleagues [3]. Compare the failed June 3 attempt with the successful July 17 one.
• Never look at the Sun to try this. If you go outside at night to look at the sky, go with an adult.

Sources

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