Astronomers in the US have unveiled a new way of measuring the mass of an exoplanet by studying several different parameters of the body's atmosphere. The technique could provide important insights into distant Earth-like objects and also help scientists to work out if life exists on other worlds. Although the technique has so far only been tested on Jupiter-sized planets, the researchers say that the next generation of telescopes will allow it to be used on objects that resemble the Earth.

Over the past decade or so, astronomers have discovered more than 900 planets orbiting stars other than the Sun, along with a further 2300 that look like exoplanets but still need to be confirmed as such. Most known exoplanets are similar to Jupiter because it is much easier to detect these giant planets using existing telescopes. Several rocky Earth-like exoplanets have, however, also been found and astronomers expect many more to be discovered using the next generation of instruments.

Wobble measurements

An exoplanet's chemical composition is expected to hold important clues about whether it could support life. This composition can be deduced from an exoplanet's density, which can be calculated by measuring its mass and radius. The mass of an exoplanet can usually be determined from the fact that as the exoplanet orbits around its parent star, the latter wobbles towards and then away from the Earth. This movement can be measured by observing the Doppler shift of the starlight as its source moves. When combined with an independent estimate of the mass of the star, an upper limit for the mass of the planet can be established.

Although this technique works well with large Jupiter-sized planets or Earth-sized objects orbiting close to very bright stars, it fails for rocky planets in orbits similar to that of the Earth round the Sun. The latter are the very exoplanets where life is most likely to exist. Now, however, Julien de Wit and Sara Seager of the Massachusetts Institute of Technology (MIT) have come up with a new way of measuring the mass that that works on "transiting" exoplanets – those that periodically pass in front of their host star.

Transiting exoplanets block some of the starlight from reaching the Earth and so, by measuring this apparent stellar dimming, the orbital period of the exoplanet as well as its diameter relative to that of the host star can be determined. However, much more additional information is also available in a transiting measurement. In particular, as a planet passes in front of its star, a small amount of the starlight travels through the exoplanet's atmosphere before reaching the Earth. Some of this light gets absorbed by the atmosphere and astronomers can already use the resulting absorption spectrum to gain important information about the chemical composition, density and temperature of the atmosphere.

Pressure drop

In this latest research, De Wit and Seager have extended this list to include information about the pressure of the atmosphere, and in particular how it changes as a function of the distance from the exoplanet's surface. Calculations made by the pair reveal that the pressure gradient, density and temperature of the atmosphere are related to the mass of the planet by a relatively simple equation. Furthermore, the researchers have shown that the pressure gradient, density and temperature can all be measured independently from the transit spectrum, thus giving the mass.

A key innovation that makes the technique possible is a new way of measuring the pressure gradient by looking at how the transparency of the atmosphere changes a function of altitude.

The researchers tested their method by using it to calculate the mass of a recently discovered exoplanet that is about 63 light-years from Earth. Called HD 189733b, this "hot Jupiter" is in a tight orbit that revolves around its parent star in just 2.2 days. Because it is an ideal candidate for existing telescopes to study, the mass of HD 189733b is known to within about 5% – it is about 1.16 times the mass of Jupiter. The new technique yielded the same mass within a similar uncertainty range.

Unfortunately, the technique only works for gas giants and cannot yet be applied to Earth-like planets using existing telescopes such as Hubble. However, De Wit and Seager believe that once Hubble's successor – the James Webb Space Telescope – comes online in 2018, the technique could then be used to study Earth-like planets because the data from the on-board spectrometers will be superior to that currently available. Planetary scientists believe that some Earth-sized exoplanets may have very different compositions from the Earth, which could have important consequences for the development of life on those worlds.

The new technique is described in Science.