Benjamin V. Rackham, PhD

I study transiting exoplanets and their host stars.


I am a 51 Pegasi b Fellow at MIT, working with Prof. Julien de Wit in the Disruptive Planets group. I earned my PhD in Astronomy & Astrophysics at the University of Arizona. In a former life, I studied neuroscience at Westminster College and worked as a field biologist in Utah and Arizona.

My Work

My work centers on finding and characterizing transiting exoplanets. I'm primarily interested in small exoplanets around nearby small stars that afford the best opportunities for detailed atmospheric studies, including searches for biosignatures in the near future. I'm interested in stellar activity in these same systems, and I work a lot on new approaches for constraining stellar photospheric heterogeneity and disentangling stellar and planetary signals in transmission spectra.

Finding new worlds

I work with some great teams to find small planets transiting small stars, including SPECULOOS, Project EDEN, and TESS. In particular, I've had the pleasure of contributing significantly to the discoveries of K2-315b, an Earth-sized planet transiting an M3.5 dwarf at 57 pc, and TOI-2406b, a sub-Neptune transiting a thick-disk M4 dwarf at 56 pc. Both are among the best known targets for atmospheric studies.

Atmospheric characterization potential for the Earth-sized K2-315b (Niraula, de Wit, Rackham et al. 2020).

Atmospheric characterization potential of the sub-Neptune TOI-2406b (Wells, Rackham et al. 2021).

Characterizing host stars

Stars and faculae can mimic or mask planetary spectral features in exoplanet transmission spectra. During my PhD, I examined the typical scale of this "transit light source" effect and found this is a concern for essentially all M-dwarf systems and many active G- or K-dwarf systems.

Change in transit depth due to unocculted spots on a typical M dwarf. The blue line shows the scale of a 100 ppm planetary feature. Spots on a typical mid-M dwarf produce signals on the same scale as planetary ones, including apparent water absorption features. (adapted from Rackham, Apai, & Giampapa 2018).

Change in transit depth due to unocculted spots for K dwarfs with typical activity levels. The blue line has the same meaning as above. Spots K dwarfs with typical activity levels can imprint apparent TiO features in transmission spectra (adapted from Rackham, Apai, & Giampapa 2019).

As a postdoc, I'm interested in new approaches to constraining the properties of exoplanet host stars and mitigating this effect. I recently co-led Study Analysis Group 21 of NASA's ExoPAG, which focused on the impact of the TLS effect on precise, space-based transmission spectra. Over the course of 18 months, this interdisciplinary group of >100 scientists identified 14 needs that can be addressed in the coming years to better constrain host-star photospheres and make the most of precise transmission spectra. These feed into seven overarching science questions, which are summarized below and detailed in full in our final report.

(Rackham, Espinoza et al. 2022)

Probing exoplanet atmospheres

Keeping in mind stellar impacts, I'm ultimately interested in studying exoplanet atmospheres, particularly in transmission, which affords us the greatest opportunity today to study small, temperate worlds. To this effect, I use HST and JWST, and I am a founding member of Project ACCESS, which is conducting a large, ground-based transmission spectroscopy survey with the 6.5-m Magellan Telescopes. To date, we've published optical transmission spectra of nine targets, including the sub-Neptune GJ1214 and the ultrahot Jupiters WASP-19b and WASP-103b. In all three of these studies, we infer the presence of unocculted photospheric heterogeneities (i.e., spots and faculae) that alter the observed transit depths, underscoring the importance of constraining stellar photospheres in order to enable unbiased studies of the planetary atmospheres.

Using three transits with Magellan/IMACS, we find that the optical transmission spectrum of GJ1214b decreases towards bluer wavelengths. The near-infrared transmission spectrum precludes a planetary atmosphere that is this transparent in the optical. We conclude that unocculted faculae on GJ 1214 are responsible for the apparently shallower optical transit depths (Rackham et al. 2017).

We measured the optical transmission spectrum of WASP-19b over six transits. After accounting for both occulted and unocculted spots and faculae in various datasets, we find the planetary transmission spectrum is flat in the optical, pointing to the presence of high-altitude clouds (Espinoza, Rackham et al. 2019).

We combined five Magellan/IMACS transits, two WHT/ACAM transits, and four archival transits to study the optical transmission spectrum of WASP-103b. The precise optical spectrum shows significant structure, which requires an implausibly high VO abundance if interpreted as a planetary spectral feature but can be readily explained as the impact of unocculted faculae in the photosphere of the F8V host star (Kirk, Rackham et al. 2021).
  • Address

    Massachusetts Institute of Technology
    77 Massachusetts Ave 54-1726
    Cambridge, MA 02319 USA
  • Email

    brackham [at] mit [dot] edu
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