How is exoplanet mass found by astrometry?

Questions in the title. Any help is appreciated :)

I know how they get the mass from radial velocity measurements but astrometry is supposed to be more accurate as the orbital inclination is not a prevailant factor.

could you just substitute reduced mass in instead of using the star mass as in the doppler case?

The newfound planet is a Jupiter-sized gas giant orbiting a star located about 500 light years from the Earth in the constellation Lyra. This world circles its star every 3.03 days at a distance of only 4 million miles (6 million kilometers), much closer and faster than the planet Mercury in our solar system.

Although such planets are relatively common, astronomers used an uncommon technique to discover it. This world was found by the "transit method," which looks for a dip in a star's brightness when a planet crosses directly in front of the star and casts a shadow. A Jupiter-sized planet blocks only about 1/100th of the light from a Sun-like star, but that is enough to make it detectable.

"This Jupiter-sized planet was observed doing the same thing that happened in June when Venus moved across (or transited) the face of our Sun," says Mandushev. "The difference is that this planet is outside our solar system, roughly 500 light years away."

To be successful, transit searches must examine many stars because we only see a transit if a planetary system is located nearly edge-on to our line of sight. A number of different transit searches currently are underway. Most examine limited areas of the sky and focus on fainter stars because they are more common, thereby increasing the chances of finding a transiting system. However the TrES network concentrates on searching brighter stars in larger swaths of the sky because planets orbiting bright stars are easier to study directly.

"All that we have to work with is the light that comes from the star," says Tim Brown (NCAR), a study co-author. "It's much harder to learn anything when the stars are faint."

Most known extrasolar planets were found using the "Doppler method," which detects a planet's gravitational effect on its star by looking for shifts in the star's spectrum, or rainbow of colors. However, the information that can be gleaned about a planet using the Doppler method is limited. For example, only a lower limit to the mass can be determined because the angle at which we view the system is unknown. A high-mass brown dwarf whose orbit is highly inclined to our line of sight produces the same signal as a low-mass planet that is nearly edge-on.

"When astronomers find a transiting planet, we know that its orbit is essentially edge-on, so we can calculate its exact mass. From the amount of light it blocks, we learn its physical size. In one instance, we've even been able to detect and study a giant planet's atmosphere," says Charbonneau.