In the past two decades alone, some 900 exoplanets – planets outside our solar system – have been identified, with some 2300 more in queue. Most of these were confirmed using the now discontinued Kepler space telescope. It’s remarkable how much scientists can find out about a distant plant, hundreds of light years away, simply by studying how light emitted by its parent star is manipulated (absorbed, reflected, tugged). For instance, researchers can establish properties like mass, planet and atmosphere composition, surface temperature and more.

As one can imagine, these readings are far from being extremely accurate. A team of researchers at MIT recently made a significant contribution to exoplanet hunting after they demonstrated a new method for assessing exoplanet mass, which they claim should be more accurate. The method is particularly useful for establishing the mass of smaller planets orbiting dimmer stars, something that currently renders skewed results using other methods. Having an accurate reading of a planet’s mass is extremely important since mass influences all the other parameters used to characterize a planet.

“The reason is that the mass of a planet is connected to its internal and atmospheric structure and it affects its cooling, its plate tectonics, magnetic field generation, outgassing, and atmospheric escape,” IT graduate student Julien de Wit said. “Understanding a planet is like dealing with a huge puzzle where knowing the mass is one of the corner pieces, which you really need to get started.”

A new way to measure mass

Artist impression of HD 189733 b and its parent star. Photo: ESA, NASA, M. Kornmesser (ESA/Hubble), and STScI

Artist impression of HD 189733 b and its parent star. Photo: ESA, NASA, M. Kornmesser (ESA/Hubble), and STScI

Typically, the mass of a planet is calculated by studying radial velocity or a measure of how intensely a planets pulls on its star. This method is useful for establishing how many planets orbit a certain star and how large these are, however it’s only accurate in certain conditions, namely for massive planets orbiting around bright star.

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The method developed by de Wit and colleagues at MIT, alled MassSpec, employs transmission spectroscopy instead. This works by measuring light from a star passing through an exoplanet’s atmosphere. A key property called pressure-scale height – how quickly the atmospheric pressure changes with altitude – is established. Then, using this data the MIT researchers can determine the planet’s gravity and, in term, mass.

A hellish world

To test the accuracy of the method, the MIT researchers looked at a gas giant HD 189733 b – a huge, Jupiter-like planet in terms of composition which orbits its parent star in only 2.2-days – previously analyzed using conventional methods. Since its a massive planet around a very bright star, measuring the exoplanet’s properties is relatively easy and accurate. After comparing the data coming from the MIT method with those from conventional methods, the results were found to be consisting.

Following the 2018 deployment of the James Webb Telescope, a multi-billion project, much powerful than Kepler, that will peer through dim and small stars, like those classed as M dwarf stars, the MIT method is sure to become truly useful. Considering there are billions of planets in the Milky Way, a new age of astronomic breakthroughs and discoveries may come out.

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