(i) Detection and mass measurement of the first isolated stellar-mass black hole:

We recently reported the first unambiguous detection and mass measurement of an isolated stellar-mass black hole (BH). We combined ground-based photometry, spectroscopy and HST astrometry of the microlensing event OGLE-11-462 to derive a lens mass of 7.1 +/- 1.5 solar mass, a distance of 1.58 +/- 0.18 kpc, and a velocity of 45 km/sec. We showed that the lens emits no detectable light, which, along with having a mass higher than is possible for a white dwarf or neutron star, confirms its BH nature (Sahu et al. 2022, ApJ, 933, 83).

(ii) Mass measurement of white dwarfs through astrometric microlensing:

We used HST to detect the deflection of a background star as Stein 2051 B passed closely in front of it. This is the first detection of the relativistic displacement by a star outside the solar system. Measurement of this deflection allowed us to determine the mass of Stein2051B as 0.675 +/- 0.051 solar masses (Sahu et al. 2017, Science, 356, 1046). We are now using this technique to measure the masses of a few more single white dwarfs using HST and JWST.

(iii) Search for planets

Through transits:

I led the SWEEPS (Sagittarius Window Eclipsing Extrasolar Planet Search) project, where we used HST to monitor a rich stellar field in the Galactic bulge to look for transiting extrasolar planets. We discovered 16 planet candidates, two of which were bright enough for radial-velocity measurements and both were confirmed as planets. The results suggest that the planets are equally abundant in the Galactic bulge and around low-mass stars (Sahu et al. 2006, Nature, 443, 1038).

Through gravitational microlensing:

Through a program which I have co-established, frequent monitoring of about 200 microlensing events towards the Galactic bulge first led to the discovery a 5.5 Earth-mass planet (Beaulieu et al. 2005, Nature, 439, 437), followed by the discovery of more than a dozen low-mass planets. These results suggest that the frequency of planets is, on average, 1.6 per star in the Milky Way, earth-like planets being more common than their more-massive counterparts (Cassan et al. 2011, Nature, 481,167).

(iv) First confirmation of extragalactic origin if GRBs:

I led a program of HST observations which showed, for the first time, that a GRB (GRB970228) was associated with an external galaxy, and that the GRB phenomenon is unrelated to the nuclear activity of the host galaxy (Sahu et al. 1997, Nature, 387, 479).

(v) Contribution of MACHOs to dark matter:

Through independent determinations of the lens locations and other statistical arguments, I have shown that the stars within the Magellanic Clouds play a dominant role as gravitational lenses and that the contribution of MACHOs to the dark matter is small (Sahu, 1994, Nature, 370, 275).

In addition, using our HST data of the SWEEPS field we have discovered, for the first time, (i) blue stragglers (Clarkson et al. 2011, ApJ, 735, 37), (ii) flare stars (Osten et al. 2012, ApJ, 754, 4), and (ii) a white dwarf cooling sequence (Calamida et al. 2014, ApJ, 790, 164), in the Galactic bulge.