Pluto got enhanced scrutiny from a rare celestial event, observed from a telescope in New Zealand on June 29, just two weeks before a NASA spacecraft flies close to the former planet. A joint Williams College-MIT-Lowell Observatory team of scientists and students traveled from Williamstown, Mass., to Lake Tekapo on the south island of New Zealand because Pluto’s shadow in starlight was calculated to fall there, in an event called an occultation (from “to occult,” meaning “to hide”).

Pasachoff, Williams scientist Bryce Babcock, colleague Robert Lucas, and Williams undergraduates Christina Seeger ‘16 and Rebecca Durst ‘17 traveled to Canterbury University’s Mt. John University Observatory’s 1-meter-diameter (40-inch diameter) telescope, supported by a grant to Williams College from NASA.

Williams College astronomer Jay Pasachoff, along with a student, had also previously observed Pluto hiding a star from that site just a year ago, but this year’s star was 100 times brighter than last year’s, so the observations are more significant. Further, observations of the way the starlight dims at the beginning of the event, and brightens at the end tells the scientists about the state of Pluto’s atmosphere. The study is relevant to understanding Earth’s atmosphere, which, like Pluto’s, is mainly nitrogen molecules, though is much, much denser.

Although Pluto has been moving farther from the Sun in its elliptical orbit for over 25 years, its atmosphere has not cooled enough for it to fall entirely to the surface as snow, leaving nothing for the NASA spacecraft to study, a situation that had not been clear before the Williams-MIT series of observations began in 2002. This year’s observations will be analyzed to find Pluto’s atmosphere’s temperature and pressure, and will be compared with the spacecraft measurements taken only two weeks later. Further, the long-term series of occultation observations, steady for over a dozen years in the past and expected to be maintained in the future, places the exquisite snapshot of Pluto’s atmosphere at one time in a long-term context. Also, Pluto’s atmosphere, though thin, is thick enough to prevent Pluto’s exact diameter from being measured from the starlight that passes through it, so New Horizons’s measurements during its July 14 near passage through the Pluto system will be valuable among other things for measuring Pluto’s diameter, which will calibrate the Williams-MIT-Lowell models for the atmosphere.

The night before the occultation, fierce winds prevented the team of astronomers and students from opening the telescope dome. But on the night of the occultation, the winds died down and the sky was clear, so the observations could be made. The Williams team was assisted by former Mt. John superintendent and staff Alan Gilmore and Pam Kilmartin. Pasachoff also arranged for Japanese scientists using an adjacent 0.6-m (24-inch) telescope, and their local collaborator, to take observations in three visible and near-infrared filters. Stephen Levine from the Lowell Observatory in Flagstaff, Ariz., made observations in the near-infrared with another adjacent 0.6-m telescope.

The Williams team was guided to their location in New Zealand by analysis of careful measurements of Pluto’s and stars’ positions made by their MIT colleagues over past years. MIT-Lowell scientist Amanda Bosh, who participated the first-ever Pluto occultation in 1988, stayed behind in Flagstaff to coordinate updates on the prediction, which were also used to make last minute changes to the flight plan of NASA’s instrumented airplane, Stratospheric Observatory for Infrared Astronomy (SOFIA). MIT scientist Michael Person was on board SOFIA to make similar measurements using a special instrument to observe occultations through the airplane’s 2.5-meter (100-inch) telescope. MIT and South African Astronomical Observatory colleague Amanda Sickafoose Gulbis used a telescope in Auckland. Other MIT students and alumni also participated.

The calculations proved to be accurate, and both Pasachoff’s ground-based observations and Person’s airplane observations showed about two minutes of occultation with not only the dimming of starlight as Pluto covered it but also a peak of brighter starlight in mid-occultation as Pluto’s atmosphere focused the starlight in a so-called “central flash.” Observing the central flash allows Pluto’s atmosphere to be probed to lower levels than are otherwise possible. In this event, the star was about seven times brighter than Pluto, so it faded considerably when it was covered by Pluto from our point of view.

Another 50 or so traveling and local professional and amateur astronomers also observed the occultation, or were defeated by clouds in their attempt. Only New Zealand and the Australian island of Tasmania were in Pluto’s stellar shadow.

The Williams-MIT-Lowell scientists and students anticipate submitting a scientific paper on preliminary results even before next week’s satellite flyby, and then to present more careful analysis to meetings of the American Astronomical Society, including its Division for Planetary Sciences, and to other astronomical professional journals.

In 2006, when an object in the outer solar system beyond Pluto was found to arguably be larger than Pluto, it became no longer tenable to classify Pluto with the planets from Mercury through Neptune. Pluto, after all, has only one-fifth of one percent of Earth’s mass, and has an orbit much stranger than those of the planets. Pluto was therefore promoted to be the brightest object in the outer solar system of a new class of object called “dwarf planets.” The term “dwarf” in astronomy is used in contrast with “giant”; the Sun, for example, is a dwarf star.

The hundreds or thousands of objects like Pluto are known as “plutoids,” another promotion for Pluto, contrary to the common statement that it was demoted by the International Astronomical Union; Pluto’s status is not even on the agenda for next month’s General Assembly of that organization in Honolulu. So Pluto, even with its atmosphere and at least the five now-known moons, will remain a “dwarf planet” for the foreseeable future.

NASA’s New Horizons spacecraft, if it survives its passage through the Pluto system, is to go on to visit another object in the outer solar system still farther away from the Earth and the Sun. The only dwarf planet in the inner solar system, Ceres, is currently being orbited by anothe
r NASA spacecraft, Dawn.

PIO Contact:
Noelle Lemoine
+1 413-597-4277
nlemoine@williams.edu

Science Contact:
Jay M. Pasachoff
+1 617-285-6351
eclipse@williams.edu

The Williams College past work on Pluto and other objects in the outer solar system is described at http://www.stellaroccultations.info

Williams College’s Hopkins Observatory, built during 1836-1838, is the oldest extant astronomical observatory in the United States. Founded in 1793, Williams College is the second-oldest institution of higher learning in Massachusetts. The college’s 2,000 students are taught by a faculty noted for the quality of their teaching and research, and the achievement of academic goals includes active participation of students with faculty in their research. Students’ educational experience is enriched by the residential campus environment in Williamstown, Mass., which provides a host of opportunities for interaction with one another and with faculty beyond the classroom. Admission decisions on U.S. applicants are made regardless of a student’s financial ability, and the college provides grants and other assistance to meet the demonstrated needs of all who are admitted.