Daniel Apai (born 1977) is a professor and astrophysicist at The University of Arizona in Tucson, Arizona. He is known for his studies of astrobiology, extrasolar planets, and the formation of planetary systems. He is the principal investigator of the team of NASA's Nexus for Exoplanet System Studies and the Hubble Space Telescope Cloud Atlas Treasury program.
Daniel Apai was born in Szeged, Hungary in 1977 and grew up in Budapest, Hungary. He studied physics at the University of Szeged, Hungary and the University of Jena, Germany, and received a diploma as research physicist in 2000. After graduation he was awarded with a German Academic Exchange Service Doctoral Fellowship and began his doctoral studies at the University of Jena, under the supervision of Thomas Henning on observational studies of young stars. In 2002 he moved to the Max Planck Institute for Astronomy, Heidelberg, Germany and he received his Ph.D. from the University of Heidelberg in 2004. In 2004 he was recipient of the Patzer Price. Between 2004 and 2008 Daniel Apai has worked as a postdoctoral researcher at the Steward Observatory's NASA Astrobiology Institute node on high-contrast adaptive optics direct imaging searches for extrasolar planets. In 2008 Apai took on a position at the Space Telescope Science Institute to work as an astronomer at the institute's Science Policy Group. In 2011 he moved back to faculty of the The University of Arizona's Steward Observatory and Lunar and Planetary Laboratory. He also held short-term visiting positions at The University of Texas and at the Max Planck Institute for Astronomy, Heidelberg.
Apai's work includes detailed comparative studies of planet formation around sun-like stars and low-mass stars; his team has discovered that the structure and evolution of protoplanetary disks depends on stellar mass. He used, for the first time, multi-epoch near-infrared radial velocity measurements to demonstrate that many O-type stars have massive companions at the time of their formation. Apai has also used the Hubble Space Telescope and the Spitzer Space Telescope to carry out pioneering observations to map condensate clouds in brown dwarf and exoplanet atmospheres. These studies demonstrated that brown dwarfs at the L- to T spectral type transition have clouds with varying thickness. Daniel Apai was also member of the team that discovered and imaged the super-jupiter Beta Pictoris b around the star Beta Pictoris and is leading the Scorpion survey that discovered and imaged the jovian exoplanet HD 131399Ab, the first long-period giant planet in a triple star system.