The transit method — observing a distant planet as it moves in front of its star as seen from Earth — is a prime tool for exoplanet detection. But transits are hardly limited to planets around their primaries. The Taiwanese-American Occultation Survey (TAOS) is demonstration of that, an attempt to find tiny Kuiper Belt objects (KBOs) in the range between 0.5 and 28 kilometers. As you would imagine, at a distance like this such objects cannot be seen directly, but an occultation — the dimming of a star when one of the KBOs passes in front of it — should be apparent. The image below shows the method applied to a cometary nucleus.
Image: Looking for dimmed starlight — the basic method at work in the TAOS survey. The experiment follows 3000 stars five times per second in its search. Credit: TAOS.
TAOS works with small, wide-field robotic telescopes on peaks near the Yu-Shan (Jade Mountain) National Park in Taiwan. The current results represent 200 hours of collected data using these instruments. And thus far TAOS has come up blank — no occultations. Perhaps the outer Solar System is not as packed with material as we had believed, an indication that smaller bodies have already merged to form the larger objects like Pluto/Charon, Eris, Haumea and their numerous cousins. Collisions may have ground smaller KBOs to a size we cannot observe, even with the equipment involved in the TAOS survey, a potential insight into the history of planetary formation in our Solar System.
The paper on this work explains the mechanisms involved in the Kuiper Belt:
The size distribution of Kuiper Belt Objects (KBOs) is believed to re?ect a history of agglomeration during the planetary formation epoch, when relative velocities between particles were low and collisions typically resulted in particles sticking together, followed by destructive collisions when the relative velocities were increased by dynamical processes after the giant planets formed… The slope of the distribution function for larger objects re?ects the early phase of agglomeration, while the shallower distribution for smaller objects re?ects a subsequent phase of destructive collisions. The location of the break moves to larger sizes with time, while the distribution for smaller objects is expected to evolve towards a steady state collisional cascade…
Given that some theories suggest a crowded Kuiper Belt, getting firm parameters on its density will be helpful. What we have thus far is that the upper size range of Kuiper Belt objects is well represented, with more than sixty bodies with a radius of over 100 kilometers having been detected. And with the help of TAOS, we now have the strongest data yet on the paucity of smaller KBOs in the range the survey is equipped to study. The work at TAOS continues, with a fourth telescope scheduled for deployment in the near future.
The paper is Zhang et al., “First Results From The Taiwanese-American Occultation Survey (TAOS),” Astrophysical Journal 685 (2008), p. L157 (abstract).
One More Thing: If you’re hoping to keep abreast of the latest Kuiper Belt news, do be aware of the Kuiper Belt Newsletter. Its stated goal: “…to provide researchers with easy and rapid access to current observational and theoretical studies of the Kuiper Belt, directly related objects (e.g., Pluto, Centaurs), and other areas of research that are explicitly applied to the Kuiper Belt.”
two thoughts.
1) 200 hours is a trivial interval considering the volume that they’re surveying and the likelihood that a shadow would fall on their observation site.
2) ‘crowded’ is probably a poor adjective. how ‘crowded’ is the asteroid belt?
At 200 hours this is far from an exhausted survey & considering the area to be covered, it probably equates to an area equal to a postage stamp in a couple of football fields being surveyed. Of course this is only an impression & I would like to know the actual percentage of the belt the survey covered. Keep looking cause it’s all out there.
What I love about science is when real data becomes available and theories must evolve. Albeit, in some areas the data is difficult to extract from the noise.
Maybe I’m missing something, but one problem I see with this is the extreme difficulty in repeating the observation to confirm it. For example, a planet transiting the star can be repeated many times to be confirmed.
In fact I understand that COROT has found many signals, but only a few have been confirmed by repeated observation.
Here, however, you get a little dimming and then nothing. The occultation depends entirely on non repeatable geometry. Maybe it could be useful as statistical tool if you could differentiate amongst various types of dimming like a planet with long period that is accidentally passing in front of the star.
The dimming pattern of a KBO and an exoplanet are sufficiently different as to be distinguishable. The researchers have modelled the pattern they expect to see to exhaustion – now they’re getting hard data.
It’s a curious finding, but collisional grinding can be surprisingly efficient at reducing small bodies into dust that’s small enough to fall into the Sun or escape.
November 7, 2008
Astronomers Discover Odd Kuiper Belt Pair
Written by Nancy Atkinson
KBO Binary. Credit: Gemini Observatory
Astronomers have discovered a pair of small Kuiper Belt Objects that are gravitationally bound to each other. This is somewhat unusual in itself. But even though these two objects are gravitationally connected, they have an enormous separation between them, about 125,000 kilometers (one third the distance from the Earth to the Moon). Astronomers say, as a comparison, this is equivalent to a pair of baseballs gravitationally “connected” and orbiting each other at a distance of 200 kilometers!
The extreme binary, 2001 QW322, orbits at 43 astronomical units or about 6.5 billion kilometers from the Sun. The pair was originally discovered in August 2001 with the Canada-France-Hawai‘i Telescope. Since then, (from 2002-2007), the pair has been monitored closely using 8-meter-class telescopes (Gemini North, Gemini South and the European Southern Observatory’s Very Large Telescope) to obtain high precision photometric observations of the faint double system.
Full article here:
http://www.universetoday.com/2008/11/07/astronomers-discover-odd-kuiper-belt-pair/
High Albedos of Low Inclination Classical Kuiper Belt Objects
Authors: M. J. Brucker, W. M. Grundy, J. A. Stansberry, J. R. Spencer, S. S. Sheppard, E. I. Chiang, M. W. Buie
(Submitted on 22 Dec 2008)
Abstract: We present observations of thermal emission from fifteen transneptunian objects (TNOs) made using the Spitzer Space Telescope.
Thirteen of the targets are members of the Classical population: six dynamically hot Classicals, five dynamically cold Classicals, and two dynamically cold inner Classical Kuiper Belt Objects (KBOs).
We fit our observations using thermal models to determine the sizes and albedos of our targets finding that the cold Classical TNOs have distinctly higher visual albedos than the hot Classicals and other TNO dynamical classes. The cold Classicals are known to be distinct from other TNOs in terms of their color distribution, size distribution, and binarity fraction.
The Classical objects in our sample all have red colors yet they show a diversity of albedos which suggests that there is not a simple relationship between albedo and color. As a consequence of high albedos, the mass estimate of the cold Classical Kuiper Belt is reduced from approximately 0.01 Earth masses to approximately 0.001 Earth masses.
Our results also increase significantly the sample of small Classical KBOs with known albedos and sizes from 21 to 32 such objects.
Comments: In press, Icarus 26 pages, 4 figures, 6 tables
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0812.4290v1 [astro-ph]
Submission history
From: Melissa Brucker [view email]
[v1] Mon, 22 Dec 2008 22:19:41 GMT (1553kb)
http://arxiv.org/abs/0812.4290
Kuiper Belt Object Occultations: Expected Rates, False Positives, and Survey Design
Authors: Steven Bickerton, Doug Welch, JJ Kavelaars
(Submitted on 19 Feb 2009)
Abstract: A novel method of generating artificial scintillation noise is developed and used to evaluate occultation rates and false positive rates for surveys probing the Kuiper Belt with the method of serendipitous stellar occultations.
A thorough examination of survey design shows that: (1) diffraction-dominated occultations are critically (Nyquist) sampled at a rate of 2 Fsu^{-1}, corresponding to 40 s^{-1} for objects at 40 AU, (2) occultation detection rates are maximized when targets are observed at solar opposition, (3) Main Belt Asteroids will produce occultations lightcurves identical to those of Kuiper Belt Objects if target stars are observed at solar elongations of: 116 deg < epsilon < 125 deg, or 131 deg < epsilon 7-8 sigma should be adopted to ensure that viable candidate events can be disentangled from false positives.
Comments: Accepted AJ, 12 pages, 12 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:0902.3457v1 [astro-ph.EP]
Submission history
From: Steven Bickerton [view email]
[v1] Thu, 19 Feb 2009 21:00:09 GMT (168kb)
http://arxiv.org/abs/0902.3457
Dynamic Resonance Effects in the Statistical Distributions of Asteroids and Comets
Authors: B.R. Mushailov, V.S. Teplitskaya
(Submitted on 2 Apr 2009)
Abstract: Some principles in the distribution of Centaurs and the “Scattered Disk” objects, as well as the Kuiper belt objects for its semi-major axes, eccentricities and inclinations of the orbits have been investigated.
It has been established, that more than a half from them move on the resonant orbits and that is what has been predicted earlier.
The divergence of the maximum in the observable distribution of the objects of the Kuiper belt for the semi-major axes with an exact orbital resonance has been interpreted.
Comments: 9 pages, 9 figures, 1 table. International Conference “100 years since Tunguska phenomenon: Past, present and future”, (June 26-28, 2008. Russia, Moscow), International Conference “Modern problems of astronomy” (August 12-18, 2007, Ukraine, Odessa)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:0904.0371v1 [astro-ph.EP]
Submission history
From: Boris Mushailov Romanovich [view email]
[v1] Thu, 2 Apr 2009 12:06:40 GMT (298kb)
http://arxiv.org/abs/0904.0371
The History of the Solar System’s Debris Disc: Observable Properties of the Kuiper Belt
Authors: Mark Booth (1), Mark C. Wyatt (1), Alessandro Morbidelli (2), Amaya Moro-Martín (3 and 4), Harold F. Levison (5) ((1) IoA, Cambridge University, UK, (2) OCA, Nice, France, (3) Centro de Astrobiologia – CSIC/INTA, Madrid, Spain, (4) Princeton University, USA, (5) SWRI, Boulder, USA)
(Submitted on 19 Jun 2009)
Abstract: The Nice model of Gomes et al. (2005) suggests that the migration of the giant planets caused a planetesimal clearing event which led to the Late Heavy Bombardment (LHB) at 880 Myr.
Here we investigate the IR emission from the Kuiper belt during the history of the Solar System as described by the Nice model. We describe a method for easily converting the results of n-body planetesimal simulations into observational properties (assuming black-body grains and a single size distribution) and further modify this method to improve its realism (using realistic grain properties and a three-phase size distribution).
We compare our results with observed debris discs and evaluate the plausibility of detecting an LHB-like process in extrasolar systems. Recent surveys have shown that 4% of stars exhibit 24 um excess and 16% exhibit 70 um excess. We show that the Solar System would have been amongst the brightest of these systems before the LHB at both 24 and 70 um. We find a significant increase in 24 um emission during the LHB, which rapidly drops off and becomes undetectable within 30 Myr, whereas the 70 um emission remains detectable until 360 Myr after the LHB.
Comparison with the statistics of debris disc evolution shows that such depletion events must be rare occurring around less than 12% of Sun-like stars and with this level of incidence we would expect approximately 1 of the 413 Sun-like, field stars so far detected to have a 24 um excess to be currently going through an LHB.
We also find that collisional processes are important in the Solar System before the LHB and that parameters for weak Kuiper belt objects are inconsistent with the Nice model interpretation of the LHB.
Comments: 16 pages, 13 figures, accepted for publication in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:0906.3755v1 [astro-ph.EP]
Submission history
From: Mark Booth [view email]
[v1] Fri, 19 Jun 2009 22:10:12 GMT (348kb,D)
http://arxiv.org/abs/0906.3755
The Creation of Haumea’s Collisional Family
Authors: Hilke E. Schlichting (1), Re’em Sari (1,2) ((1) California Institute of Technology, (2) Hebrew University)
(Submitted on 21 Jun 2009)
Abstract: Recently, the first collisional family was discovered in the Kuiper belt. The parent body of this family, Haumea, is one of the largest objects in the Kuiper belt and is orbited by two satellites.
It has been proposed that the Haumea family was created from dispersed fragments that resulted from a giant impact. This proposed origin of the Haumea family is however in conflict with the observed velocity dispersion between the family members (\sim 140 m/s) which is significantly less than the escape velocity from Haumea’s surface (\sim 900 m/s).
In this paper we propose a different formation scenario for Haumea’s collisional family. In our scenario the family members are ejected while in orbit around Haumea. This scenario, therefore, gives naturally rise to a lower velocity dispersion among the family members than expected from direct ejection from Haumea’s surface.
In our scenario Haumea’s giant impact forms a single moon that tidally evolves outward until it suffers a destructive collision from which the family is created. We show that this formation scenario yields a velocity dispersion of \sim 190m/s among the family members which is in good agreement with the observations. The probability for Haumea’s initial giant impact in todays Kuiper belt is less than 10^{-3}.
In our scenario, however, Haumea’s giant impact can occur before the excitation of the Kuiper belt and the ejection of the family members afterwards. This has the advantage that one can preserve the dynamical coherence of the family and explain Haumea’s original giant impact, which is several orders of magnitude more likely to have occurred in the primordial dynamically cold Kuiper belt compared to the dynamically excited Kuiper belt today. Abridged
Comments: Accepted for publication in ApJ, 14 pages, 1 figure
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:0906.3893v1 [astro-ph.EP]
Submission history
From: Hilke Schlichting [view email]
[v1] Sun, 21 Jun 2009 20:06:32 GMT (92kb)
http://arxiv.org/abs/0906.3893
The Collisional Divot in the Kuiper belt Size Distribution
Authors: Wesley C. Fraser
(Submitted on 1 Oct 2009)
Abstract: This paper presents the results of collisional evolution calculations for the Kuiper belt starting from an initial size distribution similar to that produced by accretion simulations of that region – a steep power-law large object size distribution that breaks to a shallower slope at r ~1-2 km, with collisional equilibrium achieved for objects r ~0.5 km.
We find that the break from the steep large object power-law causes a divot, or depletion of objects at r ~10-20 km, which in-turn greatly reduces the disruption rate of objects with r> 25-50 km, preserving the steep power-law behavior for objects at this size.
Our calculations demonstrate that the roll-over observed in the Kuiper belt size distribution is naturally explained as an edge of a divot in the size distribution; the radius at which the size distribution transitions away from the power-law, and the shape of the divot from our simulations are consistent with the size of the observed roll-over, and size distribution for smaller bodies. Both the kink radius and the radius of the divot center depend on the strength scaling law in the gravity regime for Kuiper belt objects.
These simulations suggest that the sky density of r ~1 km objects is ~10^6-10^7 objects per square degree. A detection of the divot in the size distribution would provide a measure of the strength of large Kuiper belt objects, and constrain the shape of the size distribution at the end of accretion in the Kuiper belt.
Comments: 32 pages, 10 figures, accepted to the Astrophysical Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:0910.0246v1 [astro-ph.EP]
Submission history
From: Wesley C. Fraser [view email]
[v1] Thu, 1 Oct 2009 20:18:53 GMT (319kb)
http://arxiv.org/abs/0910.0246