Exoplanetary astronomy has been one of the most productive scientific endeavors in recent years, with new discoveries of planets around other stars being announced almost on a monthly basis. Yet, apart from rough estimates concerning their mass and size, little more is known about the more than 1700 alien worlds that have been discovered to date. Despite this relative lack of more detailed information, exoplanetary research is now starting to move from mere detection to characterisation of these distant worlds. The ultimate goal is the discovery of planets with biosignatures present in their atmospheres, like oxygen, ozone, methane, and water vapor—chemical elements that are indicative of habitability.
With present-day technology, the spectroscopic study of an exoplanet’s atmosphere is a much more challenging task than the mere discovery of an exoplanet itself. Lost in the overwhelming glare of its host star, the planet’s feeble reflected light is a thousand times fainter. In addition, the blurring effects of the Earth’s atmosphere made conducting such studies with ground-based instruments in the past an almost impossible proposition. Astronomers used space-based observatories like NASA’s Hubble and Spitzer space telescope instead to conduct the first exciting spectral observations of alien atmospheres. Now, a new generation of ground-based instruments that have been developed in recent years are providing scientists with the ability to conduct such studies from astronomical observatories that are located here on Earth as well.
One such instrument expressly built for this purpose, the New Mexico Exoplanet Spectroscopic Survey Instrument, or NESSI, achieved first light earlier this month, following the successful completion of a five-year construction and commissioning period. Mounted on the Magdalena Ridge Observatory’s 2.4-m Telescope in Socorro, N.M., the multi-object spectrograph aims to bring the field of ground-based exoplanet near-infrared spectroscopy to the next level by studying the atmospheres of alien worlds in unprecedented detail in the near and short-infrared part of the electromagnetic spectrum between 1 and 2.4 μm. Conceived as a collaborative effort between the New Mexico Tech, which oversees the Magdalena Ridge Observatory and NASA’s Jet Propulsion Laboratory, in Pasadena, Calif., the $3.5 million project was awarded with a NASA Experimental Program to Stimulate Competitive Research, or EPSCoR grant, for $732,000 in September 2009. “NASA took a chance on us that we could do this,” says Dr. Michelle Creech-Eakman, an associate professor of Physics at the New Mexico Tech and Principal Investigator for NESSI. “The exoplanet field of study is moving quickly. We are studying them more and more every year – pending eventual missions to explore them. The field sparks the interest of the general public. What do these worlds look like? Could these exoplanets have the sort of atmosphere that could foster or indicate life?”
Aiming to answer these questions, astronomers will use NESSI to observe the atmospheres of the closest 100 transiting exoplanets to the Earth ranging in size from “Super Earths” to “Hot Jupiters,” utilising a technique called transit spectroscopy. Transiting exoplanets are those that pass in front of the face of their star, as seen from our line of sight here on Earth. When that happens, the star’s brightness drops by a small amount, depending on the relative size of the planet. If the transiting exoplanet additionally has an atmosphere, it will scatter the star’s light passing through it. By studying the spectrum of this scattered light, astronomers can identify the distinct chemical elements that comprise the planet’s atmosphere and obtain more detailed information about its physical properties, like its chemical composition, density, and temperature among other things.
With a cryogenic dewar filled with liquid nitrogen keeping its instruments at extremely low temperatures, NESSI will help astronomers to separate the star’s light into its constituent wavelengths, allowing them to look for the much fainter signature of the accompanying planet’s own reflected light. To achieve this level of sensitivity, the spectrograph will benefit from the use of a new software algorithm that has been developed to filter out noise and account for the blurring effects of the Earth’s atmosphere. In addition, NESSI’s 15 arc minutes wide field of view (approximately half the size of the Full Moon), will allow it to focus on two or more stars simultaneously. In this way, the light of an exoplanet around a certain star will act as the main target, and the other stars that will also be in the same field of view will have the role of calibration targets. Any atmospheric distortions affecting the light from these calibration targets will similarly affect the light from the exoplanet as well, allowing for the software algorithms to make the correct calculations for cancelling out the blurring effects of the Earth’s atmosphere.
This novel software technique has been developed by the JPL project team led by Dr. Mark Swain, a research scientist who has also led many studies in the past, concerning the observational characterization of exoplanets, like HD 189733b and HD 209458b among others. “These objects are too far away to send probes to, so the only way we’re ever going to learn anything about them is to point telescopes at them,” says Swain. “Spectroscopy provides a powerful tool to determine their chemistry and dynamics.” “They figured out this new technique for examining the spectra and we realized that the Magdalena Ridge Observatory 2.4-meter telescope is precisely the right sort of telescope to do it,” says Creech-Eakman, who also collaborated with Swain prior to her assignment at New Mexico Tech. “If you have an instrument that is sensitive enough, you can gather information from backlighting the exoplanet. By looking in the infrared light, we can see molecules in the atmosphere like water and methane, which are molecules associated with life on our own planet.”
Following almost five years of development, NESSI achieved first light on April 3 by successfully conducting observations of Pollux, an orange supergiant star in the northern constellation of Gemini, and Arcturus, a red giant star in the nother constellation of Boötes. Subsequent observations during the following nights confirmed that the instrument was operating as expected. “First light is always a big moment for any new instrument,” says Creech-Eakman. “The team is very excited about getting light through the system, and the performance is great.”
With the successful conclusion of its commissioning phase, NESSI will begin regular observations later in the summer. Currently the project’s team has been allocated 30 nights of observations by the Magdalena Ridge Observatory. Each star will require half a night of observations for the collection of its spectra. “NESSI is the first purpose-built spectrometer to measure exoplanet transit spectroscopy,” says Creech-Eakman. “There has been lots of excitement and interest about NESSI from groups of scientists interested in measuring exoplanet atmospheres, and so that is our main goal. Others will undoubtedly want to try other science, so how much gets done with NESSI will depend on getting time on the 2.4-meter telescope.”
With NESSI’s high sensitivity in obtaining transit spectroscopy measurements, the project’s team remains cautiously optimistic that exciting discoveries might lie ahead. “One question we have burning within us is, ‘Are we alone in the universe?’” says Dr. Penelope Boston, an Associate Professor in the Earth & Environmental Sciences Department at the New Mexico Tech and member of the NESSI team. “Life on other planets – that’s the perfect all-encompassing scientific question,” adds Creech-Eakman. “If we see planets with different chemistries, what would that change? What if we found a planet with bacterial life?”
Hopefully, with the start of observations by NESSI and other next-generation ground-based planet-hunter instruments that have recently become operational, like the Gemini Planet Imager and the Magellan Adaptive Optics System, we may soon start answering some of these age-old, fundamental questions of human existence.