A World on the Edge
By Dr. Mike Brown
On 15 March 2004, astronomers from Caltech, Gemini Observatory, and Yale University announced the discovery of the coldest, most distant object known to orbit the Sun. The object was found at a distance 90 times greater than that from the Sun to the Earth -- about 3 times further than Pluto, the most distant known planet.
The discovery was made on the Samuel Oschin Telescope at the Palomar Observatory east of San Diego on 14 November 2003 by the team of Mike Brown (Caltech), Chad Trujillo (Gemini Observatory) and David Rabinowitz (Yale).
Because of its frigid temperatures, the team has proposed that the object be named in honor of Sedna, the Inuit goddess of the sea from whom all sea creatures were created. Officially, the object is currently known to astronomers as 2003 VB12, based on the day of its discovery.
How far away is Sedna?
Sedna is the most distant solar system object ever discovered. It is twice as far from the Sun as any other solar system object and three times farther than Pluto or Neptune. Standing on the surface of Sedna, you could block the entire Sun with the head of a pin held at arm's length.
Even more interestingly, the orbit of Sedna is extreme elliptical, in contrast to all of the much closer planets, and it takes 10,500 years to circle the Sun.
What is the Oort cloud and what is its relationship to Sedna?
The Oort cloud is a hypothetical shell of icy proto-comets in very loose orbits around the Sun that extends to a distance of almost halfway to the nearest star. Occasionally, passing stars cause a slight change in the orbit of one of these proto-comets which causes them to come streaking in to the inner solar system where we see them as comets.Though the Oort cloud has never been seen directly, the comets that we do see are very strong evidence of its existence. The Oort cloud is expected to be much further out than the orbit of Sedna. So why do we think Sedna is a member of the Oort cloud? We believe that the existence of Sedna is evidence that the Oort cloud actually extends much further in towards the Sun than previously thought. This "inner Oort cloud" was formed in the same manner as the previously known "outer Oort cloud." Early in the history of the solar system many small icy bodies were orbiting the Sun and getting sling-shot out by close encounters with planets. As they were travelling further and further from the Sun, the orbits of these bodies were affected by distant stars, causing them to slow down and stay attached to the Sun. Sedna probably suffered a similar fate, except the stars which affected it must have been much closer than previously expected. We believe that this is evidence that the Sun formed in a tight-knit group along with many other stars.
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How big is Sedna?
In our discovery images, we see only a point of light. We can't directly measure the size of Sedna from this point. The light that we see has travelled from the Sun, been reflected off the surface of Sedna, and come back to us where we can see it in the images like the discovery images below. So a small icy object and a large coal-covered object, for example, would both look about the same brightness in the discovery images, because both objects could reflect about the same amount of Sunlight.
We can measure Sedna's size using a thermal telescope, which measures the heat coming from the surface. We know how far away Sedna is, so we know that the surface temperature is about 400 degrees below zero Fahrenheit. A large object of that temperature will give off much more heat than a small object of that temperature (just light a match and a bonfire are the same temperature, but a bonfire keeps you much warmer at night because it is so much bigger). In collaboration with Frank Bertoldi at the MPIfR Bonn, we used the 30 meter diameter IRAM telescope, and in collaboration with John Stansberry at the University of Arizona and Bill Reach at the Spitzer Science Center, we used the Spitzer Space Telescope. Sedna was too small to be detected in either. This tells us that Sedna is at most about 1800 km in diameter: about halfway in size between Pluto and the largest known Kuiper belt object Quaoar. Even though all we know for certain is that Sedna is smaller than 1800 km, we have evidence which suggests that the size might be pretty close to this number. We are virtually certain that the size is larger than the 1250 km size of Quaoar, though this object has shown many unexpected characteristics, so we can't completely rule out a smaller size.
Is Sedna a planet?
No, at least not by our definition. Astronomers have been unable to agree on a precise definition of planet, but we have a suggestion for a definition below. By our definition, Sedna is not a planet. Nor is Pluto.
What is the definition of a planet?
It is difficult for scientists to have to define a word that everybody thought they already knew the meaning of. But discoveries such as Sedna, Quaoar, 2004 DW are blurring the line between planets, asteroids, and comets. These objects are all big, so what are they? We prefer to call them planetoids. To us, a planetoid is any round object in the solar system that is not big enough to be considered a planet (actually we don't know that any of these objects are round, but it is a reasonable assumption).
So what is a planet? We define a planet to be any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit. For example, many asteroids cross the orbit of the Earth. Yet the Earth is more massive than all of those put together. Thus, the Earth is a planet. Ceres, the largest asteroid, is not greater in mass than the sum of the masses of the remaining asteroids. Hence, not a planet.
What about Pluto? Pluto sits squarely in the Kuiper belt, yet is not more massive than the total of the other Kuiper belt objects. Thus -- like Ceres -- Pluto is no planet, just the largest object in its class.
Planetary demotion has happened before. When the first asteroids were discovered they were called planets, since no one knew what else to call them. As more and more discoveries piled up it was realized that the asteroids are a separate class of bodies, the planetary designations were revoked, and the asteroids were officially reclassified as "minor planets." As we learn more about the solar system our ideas have to change. The time has come for Pluto to take its rightful place as the largest Kuiper belt object. Incidentally, if we were self-interested we would argue the other side. Our discovery of Quaoar is currently considered to be that of the largest known Kuiper belt object. If Pluto were reclassified, though, Quaoar would then be demoted to second place!
Sedna is the only object known in the inner Oort cloud, but we suspect that there will be many more found and that Sedna will not dominate the mass (or even be the most massive!). Thus, to us, Sedna is not a planet.
Our definition takes our solar system from 9 planets to 8 planets.
A alternative definition promoted by astronomers is that anything in the solar system that is made round by its own gravity should be considered a planet. The definition takes the solar system from 9 planets to hundreds of planets, when you include all of the asteroids, satellites (the moon!), and Kuiper belt objects that are round.
While the final decision is really just a matter of semantics (is Australia an island or a continent? Does it really matter?), we feel that it is important for scientists to come up with a definition that fits the centuries-old perception of what a planet is. Suddenly expanding the solar system to include 100s of planets strongly conflicts with our sense that a planet is somehow special and rare. Yet scaling back to 8 requires removing Pluto from its seemingly special place. Which is better?
An final alternative is to simply define planets as the 9 now known. That is the current definition, and it requires neither adding nor subtracting to the known number. The problem is that a purely historical instead of scientific definition will inevitably cause inconsistencies. If we find something larger than Pluto, is it a planet? Historically, no. Until astronomers agree on something more sensible than historical accident, the debate will continue.
[Editor's Note: The International Astronomical Union still classifies Pluto as a planet. You can read more about their decision here]
How well is the orbit known?
We know the orbit fairly well. After finding Sedna in November 2003, we were able to trace it back in archival data to 2001. With this nearly 3 year arc, we know that the perihelion (closest approach distance) is most likely to be within about 7 AU of our 76 AU perihelion estimate. With a perihelion of 76 AU, Sedna has a 60% farther closest approach than any other solar system object. We expect that the orbit will be improved in coming weeks as people search though archival data.
Is Sedna a Kuiper belt object?
No. Sedna never enters the region of the Kuiper belt. The Kuiper belt is an icy asteroid belt just beyond Neptune. Extremely strong evidence shows that it has a rather sharp edge at 50 AU. Sedna never comes close than 76 AU. Calling Sedna an inner Oort cloud object makes much more sense.
There are some KBOs that go very far from the Sun like Sedna does, but they all have closest approach at about 35 AU. Sedna is special because it doesn't come any closer than 75 AU to the Sun. We believe that this is because of the effects of passing stars, as described above.
A second speculative explanation for Sedna's orbit is that a larger body, perhaps Mars-sized or larger could exist at around 70 AU in a circular orbit and could have caused Sedna to get thrown into its strange orbit. If such a planet existed, we would likely have already found it in our survey, though there are still a few places left to hide.
How was Sedna found?
We have been conducting an ongoing survey of the outer solar system using the Palomar QUEST camera and the Samuel Oschin Telescope at Palomar Observatory in Southern California. This survey has been operating since the fall of 2001, with the switch to the QUEST camera happening in the summer of 2003. To date we have found around 40 bright Kuiper belt objects. To find objects, we take three pictures of a small region of the night sky over three hours and look for something that moves. The many billions of stars and galaxies visible in the sky appear stationary, while satellites, planets, asteroids, and comets appear to move. Objects in the inner Oort cloud are extremely distant and so move extremely slowly.
It is moving quite slowly and is faint, much slower and fainter than the recently discovered 2004 DW, which we also found.
Vast areas of the sky have to be searched before something this unusual is found. Our search for new objects will continue for the next few years.
How bright is Sedna; can I see it?
Sedna is about 20.5 magnitudes in R, considerably fainter than 2004 DW and Quaoar. It is beyond the reach of almost all amateurs astronomers (though, interestingly, the first confirmation of the existence of Sedna was made at Tenagra Observatory, an extremely high-end amateur telescope run by Michael Schwartz in southern Arizona).
In March 2004, the location of Sedna is easily found in the evening sky to the southwest just after Sunset. It is almost directly below Mars, and forms a triangle with the very bright Venus.
What is Sedna made of?
We don't know. Because it's surface is relatively bright, from the thermal observations (see above), we might expect it to have water ice or methane ice like Charon and Pluto have. But observations from the Gemini Telescope and (in collaboration with Chris Koresko at JPL) the Keck telescope suggest that this is not true. From observations at the 1.3-m SMARTS telescope in Chile, we do know that Sedna is one of the most red objects in the solar system -- almost as red as Mars. Why? We're currently baffled.
What else do we know about Sedna?
From observations at the 1.3-m SMARTS telescope in Chile -- in collaboration with Suzanne Tourtellotte of Yale University -- we have determined that Sedna likely rotates once every approximately 40 days. Of all of the objects in the solar system, only Mercury and Venus are known to rotate more slowly. Why the slow rotation? Our hypothesis is that Sedna's slow rotation is caused by the effect of a moon! We should soon be able to confirm the existence of this moon with observations from the Hubble Space Telescope, which should be able to direct see the tiny satellite. Stay tuned.
Sedna, 2004 DW, Quaoar, 2002 AW197, why are all these new, big objects being discovered now?
Technology is the reason. Clyde Tombaugh discovered Pluto in 1930 using photographic plates, which let you look at a very wide piece of the sky, but they are not nearly as sensitive as the CCD's that we use now. (A CCD is what you will find inside most digital cameras.) The new, large objects listed above tend to be just faint enough that they would be out of range of all the older surveys for moving objects done after Tombaugh's. Today, CCD's are getting large enough and computers are getting fast enough that it is significantly easier to find these types of planetoids than it was even 5 years ago. We use a 172 Megapixel camera mounted on a robotic telescope to find these things. Even about 5 years ago, such cameras were not available, and the computing power to analyze these cameras was not quite there either.
Are there more inner Oort cloud objects like Sedna that we haven't seen?
It is very likely that there are more inner Oort cloud objects like Sedna. We have looked at only 15% of the sky before finding Sedna. As we continue to look at the sky, we may find a few more objects like Sedna. But this is only the beginning. Kepler's law states that an object on a very elliptical orbit like Sedna spends most of its time farthest from the Sun. Thus, for every Sedna we find near closest approach, there should be many more very far from the Sun that we can't see because they are so far away and faint. Also, Sedna is rather large, about 1/2 to 3/4 the size of Pluto. Most solar system populations like the Kuiper belt objects and the asteroids actually have many more smaller objects than large objects. So, for every Sedna we find that is large, there should be many more that are small that we missed because they were faint. Although it is very difficult to make predictions from one object, it seems very likely that the inner Oort cloud will have thousands of times more objects than just Sedna. It is likely that there is more mass in the inner Oort cloud than in the Kuiper belt and the asteroid belt combined.
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Why is it called Sedna?
2003 VB12 is the official temporary designation of the International Astronomical Union (IAU) Minor Planet Center, based on the year (2003) and date (14 Nov = the 22nd 2-week period of the year thus V=the 22nd letter of the alphabet. after that it is sequential based on the discovery announcement) of discovery. Once the orbit of 2003 VB12 is known well enough (probably 1 year), we will recommend to the IAU Committee on Small Body Nomenclature -- which is responsible for solar system names -- that it be permanently called Sedna. Our newly discovered object is the coldest most distant place known in the solar system, so we feel it is appropriate to name it in honor of Sedna, the Inuit goddess of the sea, who is thought to live at the bottom of the frigid arctic ocean. We will furthermore suggest to the IAU that newly discovered objects in this inner Oort cloud all be named after entities in arctic mythologies.
You can find out more about the legend of Sedna from many websites and books, including the ones listed here:
Scientific Paper describing Sedna's Discovery - http://www.gps.caltech.edu/~mbrown/papers/ps/sedna.pdf
Sedna's story - http://www.hvgb.net/~sedna/story.html
Sedna's tale - http://www.spiralgoddess.com/Sedna.html
The legend of Sedna - http://www.inuitgallery.com/sedna.shtml
The legend of Sedna the sea goddess - http://www.polarlife.ca/Traditional/myth/sedna.htm
Funding
Our search for outer solar system objects is supported by funding from the NASA Planetary Astronomy program.
[Editor's note: This article was originally published on Brown's Caltech website at http://www.gps.caltech.edu/~mbrown/sedna/].
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