The same frequency of planets inside and

outside open clusters of stars

S. Meibom, G. Torres, F. Fessin et al. Nature 499, 55–58 (04 July 2013)

Background

• Most stars and planets formed in star clusters.

• Richer and denser clusters survive (1000-10000 per cubic parsec). 95% fall apart.

• Survived clusters are hostile environment for planets.

• Statistics support that too. More than 800 planets in Sun like stars (usually Neptine size or smaller)

• Only 4 planets in clusters.

• Discovery of 2 sub-Neptune planets in a billion year old open cluster NGC 6811 (Meibom et al. 2013)

and the punchline

Details• Before Kepler, the surveys were less sensitive.

• Difficult to detect smaller planets.

• 4 planets in star clusters, all jupiter like• A jupiter in epsilon-tau in Hyades (Sato et al. 2007)• A jupiter in TYC 5409-2156-1 in NGC 2423 (Lovis and

Mayor 2007).• Theory: most Sun like stars did not posses disk

massive enough to form gas giant planets.

• Two jupiters in Beehive cluster (Prasepe, Quimm et al. 2012) in Sun like stars.

Doppler Shift method

Problem

Transit of planets

NASA’s Kepler mission

Kepler Discoveries

Discovery of Planets in NGC6811

• NGC 6811- Open cluster in constellation of Cygnus

• At 3600 light years away with size 13’ (14-20ly across)

• Age is more than a billion year.

• At birth must have consisted more than 6000 stars including 8 O-type and 125 B type stars

• Hosts many mid F to early K stars.

Discovery of Planets in NGC6811

• Survey of 377 stars

• Found planets in Kepler-66 and Kepler-67.

Kepler-66 and Kepler-67 are indeed part of NGC 6811.

• Common Space motion of stars in NGC 6811.

• NGC6811 members radial velocity peak 7.7+/-0.8 km/s with 5 year of monitoring.

• Kepler-66 and Kepler-67 7.8+/-0.2 and 8.2+/-0.2km/s based on 7-8 measurements.

S Meibom et al. Nature 000, 1-4 (2013) doi:10.1038/nature12279

The colour–magnitude and colour–period diagrams forNGC6811.

S Meibom et al. Nature 000, 1-4 (2013) doi:10.1038/nature12279

Transit light curves.

Properties of planets in NGC6811

Validation

• Rule out false positives. No presence of nearby object in the analysis of centroid motion and high resolution imaging.

• Low precision radial velocities with Keck and MMT rule out star or brown dwarf.

• High precision Doppler spectroscopy not possible because stars very faint (15.3 and 16.4).

• BLENDER technique used to discard false positive.

BLENDER• Simulate large number of foreground background

scenarios.

• Use constraints from centroid motion, high resolution imaging and high resolution spectroscopy, color of the star, mass of the star etc.

• Found that false positives resulting from eclipsing binaries placed at distanced much farther away from cluster.

• Large planet also ruled out.

• Two false positive chances only 0.2%. One false positive chances 7.7%.

S Meibom et al. Nature 000, 1-4 (2013) doi:10.1038/nature12279

Distribution of planetary properties.

Significance

• Cluster is placid environment for planet formation, especially old cluster like NGC 6811.

• As cluster ages it dissolves away 90% of stars in first few tens of million years.

• This means NGC6811 must be initially dense cluster i.e. even more hostile environment.

• Planet formation is robust

Radial Velocity Method

Kepler’s Field of View

Statistics

• Lower limit to frequency of detectable neptunes in this cluster=1/377 =2.7E-3

• Frequency of detectable neptunes in field stars by Kepler: 6.6E-3 ((985-66)/138253)).

• Clusters should have at least 40% detectable neptunes.

• Mass of the planets was constrained based on the model, as to this size planet can have what kind of composition <10-15 earth.