[ExI] Scientists Find Solar System Like Ours

Amara Graps amara at amara.com
Fri Feb 15 18:26:32 UTC 2008

What? You didn't like my Valentine's Day present?

If you didn't notice.. try here:

"Alien Planetary System Looks a Lot Like Home"

Yeah, yeah, I know, you're thinking ...

"250 extrasolar planets known ... now another one ... ho hum..."

OK, I'll tell you why this discovery: "OGLE-2006-BLG-109"
(to be precise: "OGLE-2006-BLG-10Lb" and "OGLE-2006-BLG-109Lc")

is so cool:

1) It represents a SCALED version of our solar system with a
less-massive host star.

It found planets of mass:
	2/3 M_jupiter AND 1/2 M_saturn orbiting at distances of
	about 2.3 and 4.6 AU

	(in our own solar system we have M_jupiter and M_saturn
	orbiting at distances of 5 and 10 AU)

- Equilibrium temperatures of T ~82K and T ~59 K

	(30% less than Jupiter and Saturn)

- Orbiting around a reddish star of mass 1/2 M_sun

2) It was found by MICROLENSING, not by Doppler methods. This means
that [1]

- one doesn't need to wait for the planet to complete an orbit
- lensing zone is typically between 1.5 - 6 AU (AU = distance: Earth to Sun)
- sensitive (i.e. large signal to noise) to _low mass_ planets
- can pick up the photometry in relatively small-aperture telescopes
(even one's backyard 10 inch 'scopes)

_These planets could _not_ have been detected by the Doppler technique_

3) Amateurs (who are more sophisticated than their name implies) picked
up this object from the OGLE Early Warning System in March 28, 2006 (*)
on the MicroFUN and RoboNet network and when the lens event deviated
from a binary to a single-lens on April 5, 206, astronomers could have a
model of the planetary companion to the primary star lying near the
Einstein ring. On April 5/6, 2006, an additional peak occurred,
indicating a second planet.  Check out the long list of authors. :-)

(*) my birthday, celebrating in Turkey for the solar eclipse!

4) Higher order effects from this unually long event (week, while the
planet-detection events are usually hours) allowed the astronomers to
extract more detailed information on the radius of the planets, the
orbital motion the outer planet.

5) These are only the 5th and 6th planets to be detected by
microlensing. Given its ease of detection, such solar-system-like
planetary systems could very well be common!


[1] http://physicsworld.com/cws/article/print/19575
Gravitational microlensing

<begin quote>

Planets close to the lens star act like smaller gravitational lenses
that can briefly increase or decrease the magnification of the lens. A
cool planet in the "lensing zone" - which is typically between 1.5 and 6
times the Earth-Sun distance - can therefore be detected without having
to wait for it to complete its orbit. Furthermore, both the duration and
probability of planetary-lensing events scale as the square root of the
mass of the planet, which means that the technique is also sensitive to
low-mass planets.

Large planets like Jupiter, for example, have a 10% probability of being
in the right place to act as lenses for a few days, while Earth-mass
planets have a 1% probability of alignment and only act as lenses for a
few hours. Unlike other methods, the magnification signal in a
microlensing event can be large even though the planet is small.
However, the finite angular sizes of the source stars mean that
gravitational microlensing is not sensitive to planets smaller than

[2] Paper appeared today in Science

Discovery of a Jupiter/Saturn Analog with Gravitational Microlensing
B. S. Gaudi, D. P. Bennett, A. Udalski, A. Gould, G. W. Christie, D.
Maoz, S. Dong, J. McCormick, M. K. Szymanski, P. J. Tristram, S.
Nikolaev, B. Paczynski, M. Kubiak, G. Pietrzynski, I. Soszynski, O.
Szewczyk, K. Ulaczyk, L. Wyrzykowski, The OGLE Collaboration, D. L.
DePoy, C. Han, S. Kaspi, C.-U. Lee, F. Mallia, T. Natusch, R. W. Pogge,
B.-G. Park, The µFUN Collaboration, F. Abe, I. A. Bond, C. S. Botzler,
A. Fukui, J. B. Hearnshaw, Y. Itow, K. Kamiya, A. V. Korpela, P. M.
Kilmartin, W. Lin, K. Masuda, Y. Matsubara, M. Motomura, Y. Muraki, S.
Nakamura, T. Okumura, K. Ohnishi, N. J. Rattenbury, T. Sako, To. Saito,
S. Sato, L. Skuljan, D. J. Sullivan, T. Sumi, W. L. Sweatman, P. C. M.
Yock, The MOA Collaboration, M. D. Albrow, A. Allan, J.-P. Beaulieu, M.
J. Burgdorf, K. H. Cook, C. Coutures, M. Dominik, S. Dieters, P. Fouqué,
J. Greenhill, K. Horne, I. Steele, Y. Tsapras, From the PLANET and
RoboNet Collaborations, B. Chaboyer, A. Crocker, S. Frank, and B.
Macintosh (15 February 2008)
Science 319 (5865), 927. [DOI: 10.1126/science.1151947]

Searches for extrasolar planets have uncovered an astonishing diversity
of planetary systems, yet the frequency of solar system analogs remains
unknown. The gravitational microlensing planet search method is
potentially sensitive to multiple-planet systems containing analogs of
all the solar system planets except Mercury. We report the detection of
a multiple-planet system with microlensing. We identify two planets with
masses of ~0.71 and ~0.27 times the mass of Jupiter and orbital
separations of ~2.3 and ~4.6 astronomical units orbiting a primary star
of mass ~0.50 solar mass at a distance of ~1.5 kiloparsecs. This system
resembles a scaled version of our solar system in that the mass ratio,
separation ratio, and equilibrium temperatures of the planets are
similar to those of Jupiter and Saturn. These planets could not have
been detected with other techniques; their discovery from only six
confirmed microlensing planet detections suggests that solar system
analogs may be common.


Amara Graps, PhD      www.amara.com
Research Scientist, Southwest Research Institute (SwRI), Boulder, Colorado

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