To date, astronomers have confirmed the existence of over 900 exoplanets–alien worlds in orbit around distant stars beyond our own Solar System–as well as over 2,300 potential faraway worlds! In order to determine if any of these remote planets are habitable, astronomers must determine the mass of the planet, because this can reveal to them whether the planet is composed of gas or rock, or some other materials that may be life-friendly. In December 2013, two planet-hunting astronomers from the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts, announced that they have developed a new technique for determining the mass of distant alien worlds. Julien de Wit is lead author of the paper published on December 19, 2013 in the journal Science, along with co-author Dr. Sara Seager, the Class of 1941 Professor of Physics and Planetary Science at MIT.

The two scientists used only the exoplanets’ transit signal–the dips in starlight that occur as the wandering alien world passes in front of the face of its fiery, glaring parent-star. This information has traditionally been used to calculate the exoplanet’s size and atmospheric composition–but the MIT team has discovered a way to interpret it so that it also shows the exoplanet’s mass.

“With this method, we realized the planetary mass–a key parameter that, if missing, could have prevented us from assessing the habitability of the first potentially habitable Earth-sized planet in the next decade,”explained de Wit in a December 19, 2013 MIT Press Release. De Wit is a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Science.

Astronomers have been spotting alien worlds in orbit around Sun-like stars since 1995. Some of these alien worlds bear an eerie resemblance to the familiar planets dwelling in our Sun’s own family, while others are so weird that they do not resemble anything astronomers had ever previously dreamed of seeing. Nevertheless–despite the obvious siren’s song sung by those worlds which are enchantingly exotic–a familiar planet just like our Earth always remains the Holy Grail in planet-hunting astronomers’ search for distant worlds elsewhere in our Milky Way Galaxy.

Indeed, scientists have been observing smaller and smaller and smaller alien worlds ever since the historic discovery in 1995 of a gigantic planet in orbit around a Sun-like star. That first-of-its-weird-kind exoplanet, 51 Pegasi b (51 Peg b), was classified as a hot Jupiter. Hot Jupiters are a newly recognized class of alien worlds that orbit their parent-stars fast and close in extremely tight, roasting orbits. Until 51 Peg b was spotted, astronomers did not even suspect that such bizarre planets could exist in the Cosmos. Before 1995, astronomers believed that gigantic planets could only circle their parent-stars at much greater, colder distances–such as the more remote, chilly outer regions of our own Solar System, where Jupiter, Saturn, Uranus, and Neptune twirl majestically around our Sun.

Now, instead of merely spotting alien worlds beyond our Sun’s family, scientists want to study them in more detail. This will help them to find out if the exoplanet they are analyzing is potentially habitable.

Knowing the mass of a distant exoworld can not only enable scientists to decipher its atmospheric makeup, and whether it is a gassy or rocky planet, but can also lend some insight into its plate tectonics, how it generates magnetic fields, how it cools, and whether gas flies off from its atmosphere into space, de Wit and Seager noted.

“The mass affects everything on a planetary level. If you don’t get it, there is a large part of the planet’s properties that remains undetermined,” de Wit noted in the December 19, 2013 MIT Press Release.

In order to measure the mass of an alien world many light-years away, astronomers normally look for their parent-star’s wobble. The wobble results from the fact that planets do not actually circle their parent-stars. The planet and its star circle a mutual center of mass in their orbits–that is, the star orbits too, but in a much smaller circle. In fact, the mutual center can exist within the parent-star itself.

The stellar wobble is easiest to observe when massive planets, such as 51 Peg b, circle fast and close to their stars. Alas, the tinier the planet, the tinier the wobble induced on the star. Furthermore, scientists cannot yet measure a star’s slowest sway. This means that current methods used for weighing distant worlds are limited. The wobble method–more technically termed the radial velocity method–doesn’t work on a significant number of exoplanets that do not visibly tug on their stellar parents. This includes exoworlds with low masses, those circling their stars in more distant orbits, those dwelling in the families of dim stars, and those orbiting extremely active stars where a planet’s tugging can be masked by stellar disturbances–such as starspots.

Now, de Wit and Seager have discovered a way to weigh a distant exoplanet by observing its atmosphere.

Weighing The World!

An alien planet’s atmosphere grows thinner and thinner with altitude–just as Earth’s does. This happens because the strength of a planet’s gravitational attraction diminishes with greater distance from the planet.

Because the strength of a planet’s gravitational lure depends on its mass–the greater the mass, the stronger the pull–scientists can calculate an alien world’s mass by observing how the planet’s atmosphere thins with altitude. This involves observing exoplanets as they transit past the glaring face of their fiery stellar parent, and then gazing at the starlight gleaming through the atmospheres of those distant worlds in order to determine how the atmospheric pressure drops with altitude. Of course, this method can only work on exoplanets that have an atmosphere!

Using large space telescopes such as NASA’s venerable Hubble Space Telescope (HST) and the infrared Spitzer Space Telescope, astronomers have been able to determine the transmission spectra of newly discovered alien worlds. A transmission spectrum is generated as an exoplanet transits in front of the face of its stellar parent, allowing some starlight to shine through the atmosphere. By studying the wavelengths of the starlight that passes through, astronomers can determine an exoplanet’s atmospheric attributes–such as its temperature and the density of atmospheric molecules. From the total amount of light blocked, they can calculate an exoplanet’s size.

In order to determine the mass of an alien world using transmission spectroscopy, de Wit relied on the effect that an exoplanet’s mass has on its atmosphere. In order to accomplish this, he worked from a standard equation describing the effect of an exoplanet’s gravitational force, temperature, and atmospheric density on its atmospheric pressure profile–that is, the extent to which pressure is altered throughout the atmosphere.

According to this equation, knowing any three of these parameters would uncover the fourth parameter. Since an alien world’s mass can be derived from its gravitational force, de Wit determined that this planet’s mass could be derived from its atmosphere temperature, pressure profile, and density–which are parameters that, in principle, may be derived from a transmission spectrum.

However, in order to get a precise measurement of an alien world’s mass, de Wit needed to demonstrate that these three parameters could be derived independently of each other, only from a transmission spectrum.

In order to do this, de Wit had to show that each parameter has a distinctive effect on a transmission spectrum. De Wit then performed new analytical derivations from the first principles of radiative transfer, and discovered that an 18th-century mathematical constant–the Euler-Mascheroni constant–aids in discovering the individual effects of each parameter. This basically means that this constant acted as an “encryption key” to decode the process by which the attributes of an alien world’s atmosphere are embedded in its transmission spectrum.

In order to test this new method, de Wit used the technique on a newly spotted alien world, dubbed 189733b, that dwells 63 light-years away from Earth. From his calculations, de Wit derived the same mass measurement as that obtained by others using the radial velocity (wobble) method.

By using future high-resolution space telescopes, such as the upcoming James Webb Space Telescope–with an instrument created to observe the atmospheres of alien worlds–de Wit and Seager demonstrated that their new method will be able to characterize the mass and atmospheric properties of smaller, Earth-sized exoplanets.

“It really helps you unlock eveything and reveal, out of these crazy equations, which atmospheric properties do what, and how. You find this constant in a lot of physical problems, and it’s fun to see it reappearing in planetary science,” de Wit said in the December 19, 2013 MIT Press Release.

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