March 31st: The Informative Dwarf Planet Haumea

Date: March 31, 2009

Title: The Informative Dwarf Planet Haumea

Podcaster: Darin Ragozzine

Links: Info on Haumea:

Darin’s website:

Description: The Dwarf Planet Haumea orbits the Sun beyond Neptune and Pluto and is one of the largest known Kuiper Belt Objects. In many ways, Haumea is one of the most interesting objects in the solar system: Haumea’s ultra-fast spin stretches it out into a squashed football shape; the two moons of Haumea (Hi’iaka and Namaka) are on unusually excited orbits; and Haumea and its collisional family have somehow avoided significant space weathering. Everything we learn about Haumea is a clue to unraveling the history of the outer solar system. This podcast is an overview of this interesting and informative dwarf planet.

Bio: Darin Ragozzine is a fifth year graduate student at the California Institute of Technology, who will soon receive a doctorate degree in Planetary Science. He has spoken about Haumea at several scientific conferences and continues active scientific.research into this informative dwarf planet.

Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by Astrocamp Summer Mission of Idyllwild, California. Help introduce a child to the world of Astronomy. Learn more at

Hello. My name is Darin Ragozzine and I’m a graduate student studying Planetary Science at the California Institute of Technology in Pasadena, California. My research focuses on what is called “dynamics” or “celestial mechanics”, tracking the orbital motions of bodies in the solar system and beyond.

In many ways, this is the oldest kind of astronomy. Even the ancients knew that some of the lights in the sky moved while the majority stayed fixed. These moving stars were called “planets” from the Greek word for “wanderer” and planetary astronomy has tracked the motions of the planets for millennia. Because planets are much closer than the distant stars, they are the best astronomical objects for detailed study. When Galileo turned his telescope to the stars 400 years ago in 1609, his biggest and most important discoveries were about the planets: the phases of Venus, the moons of Jupiter, the rings of Saturn. Galileo’s legacy of what is called, “planetary astronomy” continues today in 2009, the International Year of Astronomy.

In the last decade and a half, planetary scientists like myself have had the exciting opportunity of studying the motions of newly discovered objects orbiting the sun beyond Neptune. We now know of over a thousand of these icy worlds, known as Kuiper Belt Objects or Transneptunian Objects.

The brightest known Kuiper Belt Object or KBO is Pluto. Discovered in 1930, Pluto was considered a planet for 77 years. This is one of the most popular planetary science stories in the media: Pluto’s status as a “dwarf planet” or a “planet”. With the discovery of the Kuiper belt and with the realization that Pluto is much less massive than originally thought, the International Astronomical Union voted on a definition of planet that excluded Pluto. At the same time, the IAU created a new category of objects called “dwarf planets”, which are objects large enough for gravity to make them round, but too small to clear out their orbital region of space.

Before talking more about the interesting and informative dwarf planet Haumea, I should point out that official names and categories and nice and useful, but don’t generally affect actual scientific study. And by any name, Pluto is a scientifically interesting body.

Only a few objects have been inducted into the “dwarf planet club”. In order of size, the current list of 5 official dwarf planets include Eris, Pluto, Makemake, Haumea, and Ceres, the largest asteroid. Eris, Makemake, Haumea, and other large Kuiper belt objects like Quaoar and Orcus were discovered by my Ph.D. thesis advisor, Mike Brown, at the California Institute of Technology, in collaboration with Chad Trujillo and David Rabinowitz. In fact, the “demotion” of Pluto was partly precipitated by the 2005 discovery of Eris, which is larger than Pluto in both mass and radius.

In my (biased) opinion, the most interesting dwarf planet is Haumea, formerly known as 2003 EL61. I hope to convince you in the next few minutes that Haumea is both interesting and informative, in that it provides us with unique insights into the formation of the outer solar system.

When a new bright KBO is discovered, the first thing you want to know is exactly how bright it is. Watching Haumea’s brightness was its first scientific surprise as Haumea changes in brightness by about 25% in only 1 hour. Watching the brightness variations of Haumea, David Rabinowitz and collaborators were able to calculate that Haumea rotates every 3.9154 hours, by far the fastest rotation period for large objects in the solar system. This rapid rotation stretches Haumea out into a oblong tri-axial ellipsoid shaped something like a squashed American football.

Another unique thing about Haumea is that it is orbited by two moons! While it not unique that Haumea has satellites (we’re finding that most Kuiper belt objects do have satellites), no other KBO has two satellites, except Pluto with its three satellites. Even so, Pluto’s recently discovered moons Nix and Hydra are tiny, while both of Haumea’s moons, named Hi’iaka and Namaka, are relatively large. Furthermore, Haumea’s moons have unique and exciting orbits.

But before discussing the orbits of Haumea’s moons, I want to talk a bit about how Haumea formed. The fast rotation and the moons are strong clues that Haumea as we see it today experienced a giant collision sometime in the past. The huge collision, probably one of the biggest in Kuiper belt history, contained an energy equivalent to about 10 billion nuclear bombs. As with the formation of the Earth’s Moon, part of the “splash” from the collision went into orbit and eventually formed the two moons.

Some of the pieces of Haumea went flying off into their own orbits around the Sun. This group of KBOs is known as a “collisional family” and though such families have been known in the asteroid belt for over 100 years, the Haumea family is the first and only known family in the Kuiper belt. All of the members of the Haumea family have unique surfaces made of water ice and they all orbit the sun with orbits that are the same size, shape, and orbital tilt, a textbook case of a collisional family. Haumea is clearly the origin of these objects based on its size and because we know that from its rotation and moons that it experienced a giant collision in the past.

By carefully examining the orbits of the collisional family, I was able to show that the spread of the objects indicates that the family-forming collision occurred at least a billion years ago. Other studies have shown that the most plausible story is that the Haumea collision occurred about 4 billion years ago when KBOs were about a hundred times more numerous than they are today. The collisional family of Haumea is therefore giving us clues about what the early outer solar system was like.

Now let me return to the satellites of Haumea. I have recently completed a study that determines the orbits of both Hi’iaka, the outer satellite and Namaka, the inner satellite. This was more complicated than usual because all three objects are tugging on each other, so that the orbital motion is constantly changing. Even so, the results are intriguing, to say the least. The moons are not in circular, co-planar orbits like the regular satellite systems of Pluto and the giant planets. We’re still investigating what this means for the formation and history of these moons. But most exciting of all is that the system is currently undergoing mutual events: that is, the orbit of the inner satellite Namaka is edge-on so that it crosses in front of Haumea once every 19 days.

Mutual events (otherwise known as transits, occultations, and/or eclipses) are extremely useful for improving our understanding of the Haumea system. We can’t directly watch the satellite cross in front of Haumea, but we can see the brightness of the system decrease very precisely. In the late 1980’s the mutual event season of Pluto and Charon taught us an enormous amount about these bodies. By observing Haumea’s mutual events, we will be able to get amazingly accurate measurements of the exact size, shape, and orientation of Haumea, the size and density of Namaka (the transiting moon), and more. Our group at Caltech has organized a worldwide campaign to observe these mutual events throughout the first half of 2009. As of this recording, we have already tried to observe two events and, although the results are not conclusive, it appears that we have indeed observed the predicted events.

Between an ultra-fast rotation, two unusual moons, a collisional family, and a future of mutual events, Haumea is turning out to be one of the most interesting and informative dwarf planets. Future astronomical studies of this “wandering” dwarf planet will help uncover the formation and evolution of the outer solar system, 400 years after Galileo’s first telescopic observations of these distant planets. Thanks for listening.

End of podcast:

365 Days of Astronomy
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