In a recent study published in The Astrophysical Journal Letters, a team of astronomers used the WM Keck Observatory in Maunakea, Hawai?i Island to identify an exoplanet, AF Lep b, of three times the mass of Jupiter orbiting a Sun-sized star located approximately 87.5 light-years from Earth. What makes this discovery unique is that AF Lep b is the first exoplanet discovered using a method called astrometry, which involves measuring unexpected, small changes in a star’s position relative to nearby stars. , which could indicate another object, an exoplanet, exerting a gravitational pull on its parent star.
“This is the first time this method has been used to find a giant planet orbiting a young analog of the Sun,” said Dr. Brendan Bowler, who is an assistant professor of astronomy at the University of Texas at Austin (UT Austin), and a co-author on the study. “This opens the door to using this technique as a new tool for exoplanet detection.”
In addition, the team used the direct imaging method to perform follow-up observations and confirm their initial findings. This method involves slowly blotting out the star’s glare to reveal the exoplanets orbiting it, just as you would blot out artificial light in an attempt to see objects hidden in a dark, surrounding atmosphere.
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“When we processed the observations with the Keck II telescope in real time to carefully remove the star’s glare, the planet immediately appeared and became brighter the longer we observed it,” said Kyle Franson, who is a PhD student in the Department. of Astronomy at UT Austin, and lead author of the study.
Using a combined method of astrometry and direct imaging helped the team not only confirm the existence of AF Lep b, but also determine its mass at about three times that of Jupiter, its distance from its parent star at about 8.4 Astronomical Units (AU), and its orbital eccentricity, which is the shape of the planet’s orbit, at about 0.24, measured from 0 to 1 with 0 being a perfect circle and 1 being a parabola. For context, Jupiter is 5.2 AU from our Sun, and its eccentricity is 0.05 while Earth is 1 AU from our Sun, and its eccentricity is 0.02.
Most exoplanets are discovered using the so-called transit method, which is when an exoplanet passes in front of its parent star, dimming its starlight. This method not only proves the existence of an exoplanet but can also be used to determine the exoplanet’s mass, as well.
The second most common method is the radial velocity (RV) method, which is when changes in a star’s velocity as it moves toward and away from Earth is measured as the exoplanet orbits it. This method differs from astrometry in that RV measures the speed of a star whereas astrometry measures the change in position of a star. Both of these methods are referred to as indirect methods because astronomers do not directly observe the exoplanet, only how it influences its parent star throughout its orbit.
Going forward, Dr. Bowler whose team will continue to observe and study AF Lep b.
“It will be an excellent target to further characterize with the James Webb Space Telescope and the next generation of large ground-based telescopes such as the Giant Magellan Telescope and the Thirty Meter Telescope,” said Dr. Bowler. “We are already planning a more sensitive follow-up effort at longer wavelengths to study the physical properties and atmospheric chemistry of this planet.”
What new exoplanet discoveries will astronomers make using astrometry in the coming years and decades? Only time will tell, and this is why we are science!
As always, keep doing science and keep looking up!
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