分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Recent detections of extremely short-timescale microlensing events imply the existence of a large population of Earth- to Neptune-mass planets that appear to have no host stars. However, it is currently unknown whether these objects are truly free-floating planets or whether they are in wide orbits around a distant host star. Here, we present the analysis of high-resolution imaging observations of six free-floating planet candidates collected with the Keck telescope. If these candidates were actually wide-orbit planets, the light of the host would appear at a separation of 40-60 mas from the microlensing source star. No such stars are detected. We carry out injection and recovery simulations to estimate the sensitivity to putative host stars at different separations. Depending on the object, the presented observations rule out 10-35% of potential hosts assuming that the probability of hosting a planet does not depend on the host mass. If we assume that more massive stars are more likely to host a planet, we rule out 18-63% of potential hosts. We argue that deeper observations, for example with JWST, are needed to confidently confirm or refute the free-floating planet hypothesis.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
D. Specht
R. Poleski
M. T. Penny
E. Kerins
I. McDonald
Chung-Uk Lee
A. Udalski
I. A. Bond
Y. Shvartzvald
Weicheng Zang
R. A. Street
D. W. Hogg
B. S. Gaudi
T. Barclay
G. Barentsen
S. B. Howell
F. Mullally
C. B. Henderson
S. T. Bryson
D. A. Caldwell
摘要： We present K2-2016-BLG-0005Lb, a densely sampled, planetary binary caustic-crossing microlensing event found from a blind search of data gathered from Campaign 9 of the Kepler K2 mission (K2C9). K2-2016-BLG-0005Lb is the first bound microlensing exoplanet discovered from space-based data. The event has caustic entry and exit points that are resolved in the K2C9 data, enabling the lens--source relative proper motion to be measured. We have fitted a binary microlens model to the Kepler data, and to simultaneous observations from multiple ground-based surveys. Whilst the ground-based data only sparsely sample the binary caustic, they provide a clear detection of parallax that allows us to break completely the microlensing mass--position--velocity degeneracy and measure the planet's mass directly. We find a host mass of $0.58\pm0.04 ~{\rm M}_\odot$ and a planetary mass of $1.1\pm0.1 ~{\rm M_J}$. The system lies at a distance of $5.2\pm0.2~$kpc from Earth towards the Galactic bulge, more than twice the distance of the previous most distant planet found by Kepler. The sky-projected separation of the planet from its host is found to be $4.2\pm0.3~$au which, for circular orbits, deprojects to a host separation $a = 4.4^{+1.9}_{-0.4}~$au and orbital period $P = 13^{+9}_{-2}~$yr. This makes K2-2016-BLG-0005Lb a close Jupiter analogue orbiting a low-mass host star. According to current planet formation models, this system is very close to the host mass threshold below which Jupiters are not expected to form. Upcoming space-based exoplanet microlensing surveys by NASA's Nancy Grace Roman Space Telescope and, possibly, ESA's Euclid mission, will provide demanding tests of current planet formation models.
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