Dwarf Planets: Science and Facts

Pluto was demoted to dwarf planet status in 2006, joining Eris, Haumea, Makemake and Ceres.

Credit: Karl Tate, SPACE.com

In 2006, an assembly of the International Astronomical Union voted to define a planet as a celestial body that a) is in orbit around the sun, 2) has enough mass for its gravity to pull it into a rounded shape (hydrostatic equilibrium), and 3) has cleared the neighborhood of its orbit of other, smaller objects.

This last criterion is the point at which planets and dwarf planets differ. A planet's gravity either attracts or pushes away the smaller bodies that would otherwise intersect its orbit; the gravity of a dwarf planet is not sufficient to make this happen.

Astronomers estimate that there could be as many as 200 dwarf planets in the solar system and Kuiper Belt. But the differences between planets and dwarf planets may not be obvious at first. 

Herschel’s population of trans-Neptunian objects

Herschel’s population of trans-Neptunian objects.

Credit: ESA

ESA’s Herschel space observatory has observed 132 of the known 1400 cold worlds that inhabit a region of the Solar System beyond the orbit of Neptune, some 4.5–7.5 billion km from the Sun.

These ‘trans-Neptunian objects’ (TNOs), include worlds such as Pluto, Eris, Haumea and Makemake, and make up a vast population of such objects thought to occupy these far-flung reaches of the Solar System.

TNOs are particularly cold, at around –230ºC, but these low temperatures lend themselves to observations by Herschel, which observes at far-infrared to sub-millimetre wavelengths.

Indeed, the space observatory observed the thermal emission from 132 such objects during its nearly four-year lifetime.

These measurements provided their sizes and albedos (the fraction of visible light reflected from the surface), properties that are not otherwise easily accessible.

The graphic presented here shows a sample of the population of TNOs observed with Herschel, arranged to showcase these properties.

What is most striking is their diversity. They range from just below 50 km to almost 2400 km in diameter; Pluto and Eris are the largest.

Two worlds have distinctly elongated shapes: Haumea (seen in white) and Varuna (brown). Some even host their own moons (not shown).

The albedo measurement implies a variety of surface compositions: low albedo (brown) is an indication of dark surface materials, such as organic material, while higher albedo (white) suggests pure ices.

TNOs are thought to be some of the most primitive remnants of the planet-forming era. Thus the results of the Herschel “TNOs are cool: A survey of the trans-Neptunian region” open key time programme are being used to test different models of Solar System formation and evolution.

Dwarf planet Makemake: Confirmation that it lacks atmosphere

Dwarf planet Makemake is about two thirds of the size of Pluto, and travels around the Sun in a distant path that lies beyond that of Pluto but closer to the Sun than Eris, the most massive known dwarf planet in the Solar System.

Previous observations of chilly Makemake have shown it to be similar to its fellow dwarf planets, leading some astronomers to expect its atmosphere, if present, to be similar to that of Pluto.

However, the new study now shows that, like Eris, Makemake is not surrounded by a significant atmosphere.

The team, led by Jose Luis Ortiz (Instituto de Astrofisica de Andalucia, CSIC, Spain), combined multiple observations using three telescopes at ESO's La Silla and Paranal observing sites in Chile - the Very Large Telescope (VLT), New Technology Telescope (NTT), and TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) - with data from other small telescopes in South America, to look at Makemake as it passed in front of a distant star.

"As Makemake passed in front of the star and blocked it out, the star disappeared and reappeared very abruptly, rather than fading and brightening gradually. This means that the little dwarf planet has no significant atmosphere," says Jose Luis Ortiz.

"It was thought that Makemake had a good chance of having developed an atmosphere - that it has no sign of one at all shows just how much we have yet to learn about these mysterious bodies. Finding out about Makemake's properties for the first time is a big step forward in our study of the select club of icy dwarf planets."

Makemake's lack of moons and its great distance from us make it difficult to study, and what little we do know about the body is only approximate.

The team's new observations add much more detail to our view of Makemake - determining its size more accurately, putting constraints on a possible atmosphere and estimating the dwarf planet's density for the first time.

They have also allowed the astronomers to measure how much of the Sun's light Makemake's surface reflects - its albedo. Makemake's albedo, at about 0.77, is comparable to that of dirty snow, higher than that of Pluto, but lower than that of Eris.

It was only possible to observe Makemake in such detail because it passed in front of a star - an event known as a stellar occultation.

These rare opportunities are allowing astronomers for the first time to find out a great deal about the sometimes tenuous and delicate atmospheres around these distant, but important, members of the Solar System, and providing very accurate information about their other properties.

Occultations are particularly uncommon in the case of Makemake, because it moves in an area of the sky with relatively few stars.

Accurately predicting and detecting these rare events is extremely difficult and the successful observation by a coordinated observing team, scattered at many sites across South America, ranks as a major achievement.

"Pluto, Eris and Makemake are among the larger examples of the numerous icy bodies orbiting far away from our Sun," says Jose Luis Ortiz.

"Our new observations have greatly improved our knowledge of one of the biggest, Makemake - we will be able to use this information as we explore the intriguing objects in this region of space further."