Wednesday, November 25, 2009

I stopped translating

As you may have noticed I have stopped keeping this bilingual blog long time ago.

Sunday, June 17, 2007

The Dwarf Planet Eris: larger and more massive than Pluto

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.

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.

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".

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.

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.