The parent star of one of the most distant planets known has been identified using the Hubble Space Telescope.
The planet itself was discovered indirectly three years ago, exploiting a phenomenon called gravitational microlensing. This is when one star passes in front of another, as seen from Earth. The gravity of the closer star bends the background star's light like a lens, making the background star appear to brighten and then dim over several months as they pass.
But sometimes there is second, smaller spike in brightness, indicating the presence of a planet around the lensing star. These microlensing events can reveal much more distant planets than other planet finding techniques. However, it provides very little information about the parent star itself.
Now, astronomers have for the first time managed to characterise the host star of a planet discovered by microlensing. The star is called OGLE-2003-BLG-235L/MOA-2003-BLG-53L and is 19,000 light years from Earth, making it the second most distant planetary system known. The research team was led by David Bennett of the University of Notre Dame in Indiana, US.
The lensing and background stars are still very close together in the sky - so much so that they blur together. But, because the stars are different colours, Hubble detected a slight shift in the combined object's position when observing through coloured filters.
This allowed the researchers to separately determine the properties of the lensing star and therefore to calculate more precise values for the planet's mass and size of orbit.
The researchers calculated that the host star has 63% of the Sun's mass. The planet has 2.6 times the mass of Jupiter and orbits its star at a distance of 4.3 astronomical units, similar to Jupiter's orbit in our own solar system (1 AU is the distance between the Earth and the Sun).
Although it is less massive than the Sun, the host star is heavier than the average for stars in our galaxy. This is consistent with models that predict gas giants are more common around heavier stars.
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