The Trans-Neptunian Object, and dwarf planet, Eris, charges once again. Mike Brown and Emily Schaller, published in Science the measurement of the joint mass of Eris and its moon Dysnomia through the orbital parameters of the latter (see links).
Both 10 meter Keck telescope (Mauna Kea, Hawaii) and Hubble Space Telescope (2.4 meters) were used for the determination of the orbit of Dysnomia.
The moon Dysnomia, orbits around Eris at a distance of 17430 km with a orbital period of 15.77 days. Eris and Dysnomia, together, possess a mass of 1.66x10^22 kg, which means: 27% more massive than Pluto. Given the 2400 km diameter of Eris, one concludes that it has a density of 2.3 g/cm^3. A value similar to those of the largest Trans-Neptunian Objects in contrast with the typical densities of the smallest (less that 1 g/cm^3).
These results seem to confirm that the smallest Trans-Neptunian Objects are more porous and less rich in rocky material that the largest.
Sunday, June 17, 2007
Thursday, March 15, 2007
Trans-Neptunian Objects broken into pieces
Mike Brown and collaborators discovered the first colisional family of Trans-Neptunian Objects, also known as Kuiper Belt Objects. The results had been announced by K. Barkume, Mike Brown's PhD student, in October 2006, at the Division for Planetary Sciences meeting, in Pasadena, U.S.A.
The article just came out in Nature (Brown et al. 2007, Nature, 446, 294-296). The work identifies a group of Trans-Neptunian Objects (TNOs) as the "debris" of a violent collision between two bodies. This group of TNOs was characterized by the presence of strong water ice absorption bands on its surface but with no evidence for methane ice. Note that 3 of the largest TNOs, Pluto (2320 km in diameter), Eris (2400 km) and 2005FY9 (about 1600-2000 km) possess methane ice on its surface. TNOs without methane normally show little or no water ice (we are talking about surfaces here, the interiors are another question). However, the objects: 2003EL61 (about 1500 km), 1995SM55, 1996TO66, 2002TX300, 2003OP32, 2005RR43 and S/2005(2003EL61)1 - the brightest satellite of 2003EL61 -, possess strong absorption bands due to water ice without showing methane. This fact called attention to these objects.
An orbital dynamics analysis showed that all these objects possess orbits similar to the one of 2003EL61, leading to the conclusion they are, in fact, the "debris" of the collision between the proto-2003EL61 (i.e. the old 2003EL61) and another object. The simulations suggest that 20% of the proto-2003EL61's mass can be thrown into Space if it collides with one object of 60% of its size, at about 3 km/s (10000 km/h).
The fact that 2003EL61 has a rugby ball shape, possess two satellites, a relatively high mass density and one "day" of only 4 hours indicated already that, in the past, this object had suffered a violent collision that ejected its original mantle of ices leaving it with two satellites and a stretched shape.
To get an idea of the difficulty of this discovery, let us imagine a "party" in a china store where each person has a baseball bat... In the next day someone will take a look at the pieces and to try to find out if there was a Franklin Mint Princess Diana plate there and where. The comparison is not exaggerated.
The study of Trans-Neptunian Objects, done at the sensitivity limits of the largest telescopes, continues to surprise us.
The article just came out in Nature (Brown et al. 2007, Nature, 446, 294-296). The work identifies a group of Trans-Neptunian Objects (TNOs) as the "debris" of a violent collision between two bodies. This group of TNOs was characterized by the presence of strong water ice absorption bands on its surface but with no evidence for methane ice. Note that 3 of the largest TNOs, Pluto (2320 km in diameter), Eris (2400 km) and 2005FY9 (about 1600-2000 km) possess methane ice on its surface. TNOs without methane normally show little or no water ice (we are talking about surfaces here, the interiors are another question). However, the objects: 2003EL61 (about 1500 km), 1995SM55, 1996TO66, 2002TX300, 2003OP32, 2005RR43 and S/2005(2003EL61)1 - the brightest satellite of 2003EL61 -, possess strong absorption bands due to water ice without showing methane. This fact called attention to these objects.
An orbital dynamics analysis showed that all these objects possess orbits similar to the one of 2003EL61, leading to the conclusion they are, in fact, the "debris" of the collision between the proto-2003EL61 (i.e. the old 2003EL61) and another object. The simulations suggest that 20% of the proto-2003EL61's mass can be thrown into Space if it collides with one object of 60% of its size, at about 3 km/s (10000 km/h).
The fact that 2003EL61 has a rugby ball shape, possess two satellites, a relatively high mass density and one "day" of only 4 hours indicated already that, in the past, this object had suffered a violent collision that ejected its original mantle of ices leaving it with two satellites and a stretched shape.
To get an idea of the difficulty of this discovery, let us imagine a "party" in a china store where each person has a baseball bat... In the next day someone will take a look at the pieces and to try to find out if there was a Franklin Mint Princess Diana plate there and where. The comparison is not exaggerated.
The study of Trans-Neptunian Objects, done at the sensitivity limits of the largest telescopes, continues to surprise us.
Friday, March 9, 2007
Pluto: a matter of State?
The question of the planetary status of Pluto seems to persist beyond the reasonable. The state of New Mexico (U.S.A.) intends to declare Pluto as a planet and call March 13th the "Pluto Planet Day", apparently only during this year (see the links for this news and the legislative proposal ).
Pluto's discovery was announced on March 13th of 1930, by Clyde Tombaugh. The specific announcement date was chosen as it was Persival Lowell's anniversary. Lowell had proposed the existence of a planet beyond Neptune, calling it Planet-X. Despite his efforts to detect Planet-X, founding even the Lowell Observatory (Flagstaff, Arizona, U.S.A.), he died in 1916 without observing it.
This planet definition issue may be somewhat subjective and delicate. But, actually, from the study of Pluto point of view it doesn't matter how it is classified. It is there, it is interesting, hence it is studied. The New Mexico's proposal may be funny if seen as a memorial service. But if it catches on one day we will start seeing the "Flat Earth Day", the "Geocentrism Day", or even the "Creationism Day".
Pluto's discovery was announced on March 13th of 1930, by Clyde Tombaugh. The specific announcement date was chosen as it was Persival Lowell's anniversary. Lowell had proposed the existence of a planet beyond Neptune, calling it Planet-X. Despite his efforts to detect Planet-X, founding even the Lowell Observatory (Flagstaff, Arizona, U.S.A.), he died in 1916 without observing it.
This planet definition issue may be somewhat subjective and delicate. But, actually, from the study of Pluto point of view it doesn't matter how it is classified. It is there, it is interesting, hence it is studied. The New Mexico's proposal may be funny if seen as a memorial service. But if it catches on one day we will start seeing the "Flat Earth Day", the "Geocentrism Day", or even the "Creationism Day".
Monday, February 19, 2007
The hectometer-sized Trans-Neptunian Objects, are they there or not?
In August of 2006, we have published in the Astronomical Journal the first detection of Trans-Neptunian Objects (TNOs, or Kuiper Belt Objects) with hundreds of meters of diameter: Roques et al., 2006, Exploration of the Kuiper Belt by High-Precision Photometric Stellar Occultations: First Results, AJ, 132, 819-822. This discovery implies the existence of a large population of small objects in the Solar System between 50 and 150 Astronomical Units, region that all previous astronomical observations indicated as empty. The Observatory of Paris emitted a press-release on the discovery.
Using stellar occultation techniques, i.e. the luminosity decrease of a star due to a passing object, it was possible to detect two objects beyond Neptune: one at 140 Astronomical Units (1 AU = average Earth-Sun distance, i.e. 150 million kilometers) from the Sun with 320 meters of diameter, and another one, the most distant, at 210 AU with 300 meters. A third object with 110 meters was detected at 15 AU. These objects were, also, the most distant Solar System objects detected until now.
There are many Solar System objects whose orbit exceeds 100-200 AU, however they have always been detected when they were to the Sun. Additionally, all the previously detected TNOs were detected directly, i.e. physical image of the object. Due to their very large distances and the the current limits of the world's largest telescopes all directly detected TNOs possess tens of kilometers of diameter or more.
The three announced objects were detected from the analysis of about 2 million images obtained in two nights of observations with the 4.2 meters William Herschell Telescope (WHT, La Palma, Canary Islands). Each of them with an exposure of 2 hundredth's seconds with time intervals of less than one thousandth of a second.
In that same month of August, it was published in Nature, independently, the detention of 58 Trans-Neptunian Objects with diameters under 100 m: Chang et al, 2006, Occultation of X-rays from Scorpius X-1 by small trans-neptunian objects, Nature, 442, 660-663. Detected with data from the X-ray satellite RXTE (NASA), using a similar method.
With these two works the existence of hectometer-sized TNOs seemed secure. However, it has become available in astro-ph the article Jones et al., 2007, Millisecond Dips in Sco X-1 are Likely the Result of High-Energy Particle Events (not accepted for publication, yet) stating that Chang et al.'s (2006) detections are not, in fact, small TNOs but rather "cosmic-rays" that hit the RXTE detectors.
This does not invalidate the work of Roques et al. (2006) but diminishes, for the time being, its force by invalidating the independent detections of Chang et al. (2006). Despite the possible non-detection of hectometer-sized TNOs with RXTE, I am still convinced that the detentions with WHT are real. They do exist, and others will be detected.
Using stellar occultation techniques, i.e. the luminosity decrease of a star due to a passing object, it was possible to detect two objects beyond Neptune: one at 140 Astronomical Units (1 AU = average Earth-Sun distance, i.e. 150 million kilometers) from the Sun with 320 meters of diameter, and another one, the most distant, at 210 AU with 300 meters. A third object with 110 meters was detected at 15 AU. These objects were, also, the most distant Solar System objects detected until now.
There are many Solar System objects whose orbit exceeds 100-200 AU, however they have always been detected when they were to the Sun. Additionally, all the previously detected TNOs were detected directly, i.e. physical image of the object. Due to their very large distances and the the current limits of the world's largest telescopes all directly detected TNOs possess tens of kilometers of diameter or more.
The three announced objects were detected from the analysis of about 2 million images obtained in two nights of observations with the 4.2 meters William Herschell Telescope (WHT, La Palma, Canary Islands). Each of them with an exposure of 2 hundredth's seconds with time intervals of less than one thousandth of a second.
In that same month of August, it was published in Nature, independently, the detention of 58 Trans-Neptunian Objects with diameters under 100 m: Chang et al, 2006, Occultation of X-rays from Scorpius X-1 by small trans-neptunian objects, Nature, 442, 660-663. Detected with data from the X-ray satellite RXTE (NASA), using a similar method.
With these two works the existence of hectometer-sized TNOs seemed secure. However, it has become available in astro-ph the article Jones et al., 2007, Millisecond Dips in Sco X-1 are Likely the Result of High-Energy Particle Events (not accepted for publication, yet) stating that Chang et al.'s (2006) detections are not, in fact, small TNOs but rather "cosmic-rays" that hit the RXTE detectors.
This does not invalidate the work of Roques et al. (2006) but diminishes, for the time being, its force by invalidating the independent detections of Chang et al. (2006). Despite the possible non-detection of hectometer-sized TNOs with RXTE, I am still convinced that the detentions with WHT are real. They do exist, and others will be detected.
Thursday, February 15, 2007
Colision of Comets at the Helix Nebula
[Image:NASA/JPL-Caltech/Univ. Arizona]
Kate Su (University of Arizona, U.S.A.) and collaborators, find evidence for a high number of collisions between comets at the Helix Nebula, using the
Spitzer Space Telescope (NASA).
The Helix Nebula is at about 700 light-years of distance from the Earth and it is the final phase of a star similar to the Sun: a white dwarf surrounded by a distant cloud of gas and dust.
The Spitzer Telescope operates in the infra-red, hence it does not get images in the visible like the Hubble Telescope does. However, it is capable to detect the thermal radiation from tiny close objects or from very distant objects. The different intensities of the detected radiation are transformed into false colors to create an image.
Kate Su's team detected an excess of "thermal brightness" between 35 and 150 Astronomical Units (1 AU = Earth-Sun Distance = 150 000 000 km) of distance from the white dwarf at the center of the Helix Nebula. Most probably due to a dust disk. One did not expect to find dust at such distances around this type of star. However, in our Solar System, at the same distance from the Sun, we have the Kuiper Belt. And, in reality, it is a large reservoir of comets which, simplifying, are no more than "dirty ice balls". It is then most likely that that the detected dust was released by a large number of collisions between comets that gravitate, or gravitated, around the nebula's central star.
The Spitzer press-release is available on-line.
The article's abstract: Su et al., 2007, Debris Disk around the Central Star of the Helix Nebula?, Astrophysical Journal Letters, Vol. 657, L41-L46, is also available.
Thursday, February 1, 2007
The planet definition is only for the Solar System
When changing e-mails with José Matos, I realized it is not immediately clear that IAU's definition of planet regards only to the Solar System objects. On an article in "Sky & Telescope" (November 2006 ), Owen Gingerich, from the Harvard-Smithsonian Center for Astrophysics (USA), member of IAU's committee for the planet definition, tells what happened. While trying to establish an expression for the gravitational domination of the orbital region of an object, they also restricted the definition to the Solar System only. The dynamical evolution of our Solar System is not necessarily applicable to the other planetary systems.
This issue is subject to some confusion. In 2003, IAU, through the "Working Group on Extrasolar Planets" (WGESP) emitted a declaration on the (extra-solar) planet's definition:
1) Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.
2) Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.
3) Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).
This declaration did not intend to be a canonic definition but an evolutive definition instead. It is, however, obvious that in the future a strict definition will have to be found. The story of the process involving the planet definition is quite in the Wikipedia (see: Planet; 2006 Redefinition of Planet; Definition of Planet). However, the entry "Planet" gives the illusion that IAU also created a strict definition for the extra-solar planets, when it is not the case yet.
This issue is subject to some confusion. In 2003, IAU, through the "Working Group on Extrasolar Planets" (WGESP) emitted a declaration on the (extra-solar) planet's definition:
1) Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.
2) Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.
3) Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).
This declaration did not intend to be a canonic definition but an evolutive definition instead. It is, however, obvious that in the future a strict definition will have to be found. The story of the process involving the planet definition is quite in the Wikipedia (see: Planet; 2006 Redefinition of Planet; Definition of Planet). However, the entry "Planet" gives the illusion that IAU also created a strict definition for the extra-solar planets, when it is not the case yet.
Friday, January 26, 2007
Pluto is not a planet
In 1992, Jewitt & Luu identify the first Trans-Neptunian Object: 1992QB1. The Kuiper Belt, whose existence was speculated since the 30's, had been discovered. Presently, more than 1200 objects were discovered in the Kuiper Belt. Being a member of this belt, Pluto saw its classification as planet compromised. After the discovery announcement of Eris (2003UB313) by Brown, Trujillo & Rabinowitz, in 2005, Pluto was definitely doomed. The question might seem strange but a strict definition of planet did not exist. The distinction between planet and star is still a subject of debate for the extra-solar planets. However, regarding to the Solar System we all believed to know who was and who was not a planet. A simple meeting was enough to conclude we do not know it as we would like to.
The International Astronomical Union (IAU), on its XXVI General Assembly, in Prague, from 16 to 24 of August, 2006, proposed to establish a planet definition. The initial proposal, essentially, established only the mass and sphericity as criteria to an object which was not a satellite to be a planet. In a way it was established that only size matters: Pluto would continue to be a planet. However, after a large debate it was determined to take also into account, as an additional criteria, the gravitational control of its orbital region. That is, it was also necessary to have cleared its orbital region of other objects: Pluto would not be a planet now. Also, it was decided that the planet definition would regard only to Solar System objects. The extra-solar planets continue, still, without a strict definition.
On the 24th, the Resolution 5A was approved [see IAU website]:
[...] The IAU therefore resolves that "planets" and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way: (1) A "planet" (see footnote 1) is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. (2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape (see footnote 2), (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. (3) All other objects (see footnote 3) except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".
Footnote 1: The eight "planets" are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Footnote 2: An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
Footnote 3: These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies. It was also approved the Resolution 6A:
[...] The IAU further resolves: Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects (see footnote 1).
Footnote 1: An IAU process will be established to select a name for this category."
I confess that the demotion of Pluto pleases me . If Pluto had been discovered after 1992 it would never had been classified as planet. Objectively or not, words are important. However, the new definition possesses its problems. Subjectively, the term "dwarf planet" does not seem to be a very good choice for something that it is not a planet. Objectively, items 1b-2b and 1c-2c are more vague than they seem and problems will most likely arise. Not that I have heard a better idea. The problem here may be resumed to the Sorites Paradox, attributed to Eubulides of Miletus, that Doctor Nuno Crato likes to quote. That is: at which point a heap of wheat stops being a heap if I continuously remove wheat grains?
If borderline objects between dwarf planets and small bodies exist, as probably they will, we will have situations where one day an object is a small body by 1 km of diameter and on the next day it will be a dwarf planet, due to a more accurate measurement of its diameter. The question of clearing the neighbourhood will also be delicate. What is the maximum number and the minimum size of dust / small bodies allowed in a neighborhood so that it is considered clean? Problems like these will be delightful for the miscontent with Pluto's demotion.
But... from the scientific point of view, is this question truly important? In my opinion: in theory no, in reality yes. No: because in theory Science will not be neither more nor less interested on Pluto, or any other object, due to a subjective classification. Yes: because Science also lives on social and political impacts. However, from this point of view, I think that the balance will be positive. Pluto may have lost some importance to the eyes of society but, suddenly, all the other dwarf planets had gained it.
Notes: There are a huge amount of articles and opinions on this issue. There are, however, two sites one has to visit: David Jewitt's web page, with particular emphasis on the article "On Pluto, perception & planetary politics", and Mike Brown's web page, one of the miscontent with the demotion.
The International Astronomical Union (IAU), on its XXVI General Assembly, in Prague, from 16 to 24 of August, 2006, proposed to establish a planet definition. The initial proposal, essentially, established only the mass and sphericity as criteria to an object which was not a satellite to be a planet. In a way it was established that only size matters: Pluto would continue to be a planet. However, after a large debate it was determined to take also into account, as an additional criteria, the gravitational control of its orbital region. That is, it was also necessary to have cleared its orbital region of other objects: Pluto would not be a planet now. Also, it was decided that the planet definition would regard only to Solar System objects. The extra-solar planets continue, still, without a strict definition.
On the 24th, the Resolution 5A was approved [see IAU website]:
[...] The IAU therefore resolves that "planets" and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way: (1) A "planet" (see footnote 1) is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. (2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape (see footnote 2), (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. (3) All other objects (see footnote 3) except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".
Footnote 1: The eight "planets" are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Footnote 2: An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
Footnote 3: These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies. It was also approved the Resolution 6A:
[...] The IAU further resolves: Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects (see footnote 1).
Footnote 1: An IAU process will be established to select a name for this category."
I confess that the demotion of Pluto pleases me . If Pluto had been discovered after 1992 it would never had been classified as planet. Objectively or not, words are important. However, the new definition possesses its problems. Subjectively, the term "dwarf planet" does not seem to be a very good choice for something that it is not a planet. Objectively, items 1b-2b and 1c-2c are more vague than they seem and problems will most likely arise. Not that I have heard a better idea. The problem here may be resumed to the Sorites Paradox, attributed to Eubulides of Miletus, that Doctor Nuno Crato likes to quote. That is: at which point a heap of wheat stops being a heap if I continuously remove wheat grains?
If borderline objects between dwarf planets and small bodies exist, as probably they will, we will have situations where one day an object is a small body by 1 km of diameter and on the next day it will be a dwarf planet, due to a more accurate measurement of its diameter. The question of clearing the neighbourhood will also be delicate. What is the maximum number and the minimum size of dust / small bodies allowed in a neighborhood so that it is considered clean? Problems like these will be delightful for the miscontent with Pluto's demotion.
But... from the scientific point of view, is this question truly important? In my opinion: in theory no, in reality yes. No: because in theory Science will not be neither more nor less interested on Pluto, or any other object, due to a subjective classification. Yes: because Science also lives on social and political impacts. However, from this point of view, I think that the balance will be positive. Pluto may have lost some importance to the eyes of society but, suddenly, all the other dwarf planets had gained it.
Notes: There are a huge amount of articles and opinions on this issue. There are, however, two sites one has to visit: David Jewitt's web page, with particular emphasis on the article "On Pluto, perception & planetary politics", and Mike Brown's web page, one of the miscontent with the demotion.
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