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"  The exoplanet named HD 189733 b sits about 63 light years million miles from Earth. And though scientists can&amp;#039;t see this distant planet, they know a lot about it. It&amp;#039;s a gas giant close in composition to Jupiter, but its 2.2-day orbital period around its star means the planet is scorching, with surface temperatures reaching near 2000 degrees. Its marble blue color doesn&amp;#039;t come from a boiling sea, but rather from an atmosphere of raining molten glass. In other words, HD 189733 b is a hell planet. And thanks to MIT graduate student Julien de Wit, this world will hopefully help solve future exoplanet mysteries by telling scientists how massive these far-flung planets are. The number of known exoplanets has exploded in the last two decades; today astronomers know of more than 900 exoplanets, and 2300 more candidates await confirmation. This is only a small sliver of the projected 400 billion exoplanets in the Milky Way. But, as de Wit describes in his study published today in Science, scientific priorities are slowly shifting from detecting exoplanets to characterizing them, and the most important thing to know about a new planet is its mass. &amp;quot;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,&amp;quot; de Wit says via email. &amp;quot;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.&amp;quot; That&amp;#039;s why de Wit, along with MIT professor Sara Seager, developed MassSpec, a method that can discern the mass and atmosphere of exoplanets simultaneously. The technique could help to detect planets in the habitable zone around their stars. Typically, scientists calculate an exoplanet&amp;#039;s mass using a method called radial velocity, which measures how intensely a planet pulls on its star. That in turn can tell scientists how many planets orbit a star and how large they are. But this method is accurate only in certain conditions: massive planets that orbit bright stars. MassSpec uses transmission spectroscopy, which measures light from the exoplanet&amp;#039;s parent star passing through the planet&amp;#039;s atmosphere. Then de Wit determines what&amp;#039;s called pressure-scale height, a measurement that tells researchers how quickly the atmospheric pressure changes with altitude. From this data, de Wit can determine a planet&amp;#039;s gravity and its mass. De Wit tested this technical process on the gas giant HD 189733 b, whose bright star and huge size has made cataloging its mass relatively easy. When he compared the numbers from his technique to official numbers, they were consistent, and de Wit knew he had uncovered a new way to weigh a planet. De Wit&amp;#039;s method will truly hit its stride with the planned 2018 launch of the James Webb Space Telescope (JWST), which will search for habitable planets around M dwarf stars. Because the stars are so faint, de Wit says, radial velocity won&amp;#039;t be any help in finding the mass of these planets. With JWST, MassSpec will be able to learn more about the kind of planets that are most likely to support life. "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/space/deep-space/a9858/how-to-weigh-an-exoplanet-16284612/>