Astronomers find the first exoplanet in a triple star system

An astronomer from the California Institute of Technology discovered a triple star system HD 188753, which contains a planet just a little larger than Jupiter. The gas giant is located a short distance from the main star of the system, and the other two stars, which form a close pair, orbit around this star in a remote orbit.

The HD 188753 system is located 149 light years from us, in the constellation Cygnus. The main star of the system weighs almost as much as our Sun - 1.06 solar masses. A couple of other stars, the total mass of which is 1.63 solar, orbits around it in an orbit approximately corresponding to the orbit of Saturn.
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The discovered planet is very close to the main star - its orbital period is only 3.5 Earth days. It is expected that due to such proximity the temperature of its surface should be very high.

“The situation in which this planet is located looks very impressive,” Maciej Konacki from KTI shares her observations. “With three suns at once, the view of the sky must be absolutely unearthly - both literally and figuratively.” Konaki discovered the planet in the system using Keck I - one of the Keck Observatory telescopes in Hawaii.

The explanation of the formation of such an unusual system presents certain difficulties for astrophysicists. Although the Jupiter-like planets had already been observed in such proximity to the star, the extra stellar couple in the system should have pulled the main mass of matter from the protoplanetary disk and did not allow such a large planet to form.

In addition, usually these planets are formed for the so-called. "Line of freezing". At a sufficiently large distance from the star, of the order of 3 AU, a solid core of the planet is formed, which then begins the accretion of gas from the protoplanetary disk. But the massive starry pair present in the system should have absorbed most of the matter from the disk, reducing its size to 1.3 AU, which would not allow the gas giant to form.

Exoplanets are already counted on thousands, about 30 of them belong to binary stellar systems - but the described case was the first in practice. Since about half of the visible stars in our galaxy belong to binary stars, the percentage of systems found that have planets is very low.

In particular, the reason for this is the difficulty of finding planets in binary systems by the radial velocity method. A planet orbiting a star changes its speed relative to the Earth, and the corresponding spectral changes can be measured. When it comes to a system of more than one star, changes in the spectrum become more complex.

Konaki developed a method for isolating changes due to the presence of a planet from spectrum data. And so he discovered the planet in the triple system among the first 20 systems he had viewed. In total, he plans to study about 450 known triple systems in order to search for planets.


Keck Observatory

At the Keck Observatory in Hawaii, there are two mirror telescopes, the equivalent diameter of the hexagonal primary mirrors is 10 meters. Each mirror is made up of 36 small hexagonal mirrors. These telescopes are among the largest in the world.

In 1999, one of the first adaptive optics systems was installed at the Keck Observatory, which allows to eliminate atmospheric distortion. And in 2001, an interferometer was installed there, connecting both telescopes together. Since the Keck I and Keck II telescopes are located about 85 meters from each other, this made it possible to achieve a resolution equivalent to a telescope with an 85-meter mirror, that is, about 0.005 arc-seconds.