Tali Khain (B.S. ‘19) is the 2019 winner of the prestigious American Physical Society (APS) LeRoy Apker Award, which honors excellence in undergraduate research in physics. Tali was recognized “for original contributions to understanding the outer solar system, including characterizing the dynamical properties of hundreds of new objects in the Kuiper Belt; establishing the orbital stability of a new dwarf planet; and investigating the effects of a hypothesized distant new planet.” Tali will receive a $5000 prize as well as the opportunity to give an invited talk at an APS conference. The Physics department will also receive $5000 to further support undergraduate research. Tali becomes the second U-M undergraduate to receive the Apker Award; Matthew Becker won in 2007. During her undergraduate career Tali made significant, substantive contributions to an astonishing 10 peer-reviewed publications, including 3 on which she was the first author.
Tali worked with Professor Fred Adams and Professor David Gerdes throughout her career at Michigan, starting the summer after her senior year of high school. Her work focused on the orbits of objects in the Kuiper belt, the region of the solar system beyond Neptune. She used a combination of observational data and theoretical modeling to study patterns in the motion of rocky bodies orbiting the Sun in the Kuiper belt. These bodies, called Kuiper belt objects (KBOs), usually have more elliptical orbits than the planets, and the shapes of their orbits can change gradually over time due to gravitational interactions with the outer planets, especially Neptune. Using data from the Dark Energy Survey (DES), Tali worked to classify the orbits of hundreds of KBOs discovered by the U-M team. One of her first contributions was to demonstrate that the Michigan-sized dwarf planet 2014 UZ224 (“DeeDee”), which was discovered by the U-M group in 2016, occupies an orbit that is stable over the 4.5 billion year lifetime of the solar system.
Much of Tali’s award-winning work analyzed distant objects with particularly large and elliptical orbits. These KBOs take thousands of years to complete one orbit around the sun. The orbits of these KBOs appear to all point in roughly the same direction, a surprising arrangement that is unlikely to have arisen by chance. A recent hypothesis suggests that there might be a large, undiscovered planet twenty times farther away from the Sun than Neptune, referred to as Planet Nine. Using numerical simulations of the proposed gravitational effects of Planet Nine, Tali showed that this hypothesis can help explain the orbital arrangement of the distant KBOs, and enhance the stability of their orbits.
In addition, Tali considered the consequences of two large bodies - Neptune and Planet Nine - on the orbits of the KBOs. A fascinating predicted effect of the two planets is called "resonance hopping", in which a KBO transitions from one resonance with Planet Nine or Neptune to another. An orbital resonance is a stable state in which the orbit of the KBO is tied via gravity to a much larger body. As part of her work, Tali performed simulations to understand the causes of resonance hopping in the Kuiper belt. She found that that the objects "hop" to a new resonance due to perturbations from Neptune's gravitational pull. As more data is analyzed from DES and other surveys, this work can be refined and extended in the hope that Planet Nine may be discovered in the near future.
“Without hyperbole, Tali is one of the handful of world experts on the dynamics of Planet Nine,” says Professor Gerdes. “Tali is a spectacularly gifted scientist, and it’s been a great privilege to work with her—and eventually for her. When she was only a first-year undergraduate, colleagues at other institutions were already complimenting me on the outstanding new ‘graduate student’ in my group. Her future is bright indeed, and I can’t wait to see what she will accomplish.”
Tali is continuing to pursue physics in graduate school. This fall, she began work towards a Ph.D. at the University of Chicago, studying soft matter physics.