Rocky, Low-Mass Planet Discovered by Microlensing

Universe Today – gravitational microlensing, promises to find planets down to 10 Earth masses, much further out from their parent stars. Using this technique, a team of astronomers has just announced the detection of a rocky planet just in this range.

Arxiv – Discovery and Mass Measurements of a Cold, 10-Earth Mass Planet and Its Host Star (38 pages)

Astronomers have discovered 13 planets using gravitational microlensing. The newly announced one, MOA-2009-BLG-266Lb, is estimated to be just over 10 times the mass of Earth and orbits at a distance of 3.2 AUs around a parent star with roughly half the mass of the Sun. The new finding is important because it is one of the first planets in this mass range that lies beyond the “snow line”, the distance during formation of a planetary system beyond which ice can form from water, ammonia, and methane. This presence of icy grains is expected to assist in the formation of planets since it creates additional, solid material to form the planetary core. Just beyond the snow line, astronomers would expect that planets would form the most quickly since, as you move further, beyond this line, the density drops. Models have predicted that planets forming here should quickly reach a mass of 10 Earth masses by accumulating most of the solid material in the vicinity. The forming planet then, can slowly accrete gaseous envelopes. If it accumulates this material quickly enough, the gaseous atmosphere may become too massive and collapse, beginning a rapid gas accretion phase forming a gas giant.

We present the discovery and mass measurement of the cold, low-mass planet MOA-2009-BLG-266Lb, made with the gravitational microlensing method. The planet and host star mass measurements are enabled by the measurement of the microlensing parallax e ect, which is seen primarily in the light curve distortion due to the orbital motion of the Earth. But, the analysis also demonstrates the capability to measure microlensing parallax with the Deep Impact (or EPOXI) spacecraft in a Heliocentric orbit. The planet mass and orbital distance are similar to predictions for the critical core mass needed to accrete a substantial gaseous envelope, and thus may indicate that this planet is a failed” gas giant. This and future microlensing detections will test planet formation theory predictions regarding the prevalence and masses of such planets.

We expect the rate of these survey discoveries to increase quite rapidly in the near future as the number of telescopes involved in these high cadence surveys is increasing quite rapidly. The OGLEIV survey with a 1:4 deg2 camera has just begun on the OGLE 1.3m telescope in Las Campanas, Chile. Although OGLE-IV has a smaller telescope and eld of view than MOA-II, it has better seeing, and so it should have higher planet detection sensitivity. Nearly complete longitude coverage should also be possible for part of the season as a group from Tel-Aviv University is beginning a dedicated Galactic bulge monitoring program with a 1:0 deg2 imager on the 1.0m telescope at Wise Observatory in Israel in 2011 after a 6-week pilot program in 2010.

The most ambitious project is the Korea Microlensing Telescope Network (KMTNet) (Kim et al. 2010), which is building a network of three 1.6 m telescopes equipped with 4:0 deg2 cameras in South Africa, (northern) Chile, and Australia. The KMTNet system will have the capability for continuous coverage of all bulge microlensing events by itself, when the weather permits, but it is also locating its telescopes at di erent sites from the existing MOA-II, OGLE-IV, and Wise telescopes, so that complete light curve coverage will often be possible when some sites have bad weather. This should result in a signi cant increase in the rate of microlensing planet discoveries

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