MIT Scientists develop new technique to measure mass of exoplanets

Artistic rendering of a planet's transmission spectrum. 

Credit: CHRISTINE DANILOFF /MIT, JULIEN DE WIT

To date, scientists have confirmed the existence of more than 900 exoplanets circulating outside our solar system.

To determine if any of these far-off worlds are habitable requires knowing an exoplanet's mass—which can help tell scientists whether the planet is made of gas or rock and other life-supporting materials.

But current techniques for estimating exoplanetary mass are limited. Radial velocity is the main method scientists use: tiny wobbles in a star's orbit as it is tugged around by the planet's gravitational force, from which scientists can derive the planet-to-star mass ratio.

Spitzer Space Telescope

For very large, Neptune-sized planets, or smaller Earth-sized planets orbiting very close to bright stars, radial velocity works relatively well.

But the technique is less successful with smaller planets that orbit much farther from their stars, as Earth does.

Now scientists at MIT have developed a new technique for determining the mass of exoplanets, using only their transit signal—dips in light as a planet passes in front of its star.

Julien de Wit

This data has traditionally been used to determine a planet's size and atmospheric properties, but the MIT team has found a way to interpret it such that it also reveals the planet'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—will actually be accessible, together with its atmospheric properties," says Julien de Wit, a graduate student in MIT's Department of Earth, Atmospheric and Planetary Sciences.

De Wit is lead author on a paper published today in the journal Science, with co-author Sara Seager, the Class of 1941 Professor of Physics and Planetary Science.


Researchers at MIT explain what exactly an exoplanet or extrasolar planet is, why we study them and how you can detect them.

"The mass affects everything on a planetary level, such as any plate tectonics, its internal cooling and convection, how it generates magnetic fields, and whether gas escapes from its atmosphere," de Wit says.

"If you don't get it, there is a large part of the planet's properties that remains undetermined."

Using large telescopes such as the NASA's Spitzer and Hubble Space Telescopes, scientists have been able to analyze the transmission spectra of newly discovered exoplanets.

A transmission spectrum is generated as a planet passes in front of its star, letting some light through its atmosphere.

By analyzing the wavelengths of light that pass through, scientists can determine a planet's atmospheric properties, such as its temperature and the density of atmospheric molecules. From the total amount of light blocked, they can calculate a planet's size.

More information: "Constraining Exoplanet Mass from Transmission Spectroscopy," by J. de Wit et al. Science, 2013. DOI:10.1126/science.1245450

New technique for enhanced reversibility of a phase-transforming Martensite material

Various hierarchical micro-structures in Au30, the alloy most closely satisfying the cofactor conditions for both type I and type II twin systems. Credit: Nature 502, 85–88 (03 October 2013) doi:10.1038/nature12532

A team of researchers at Minnesota State University has created a martensitic metal alloy that degrades very little after application of more than 16,000 thermal cycles.

In their paper published in the journal Nature, the team describes how they applied mathematical theory to a zinc-gold-copper alloy and found it markedly improved the alloy's ability to withstand degradation due to multiple heating-cooling cycles.

Metals that return to a previous physical shape after being changed (bent, crumpled, etc.) using certain techniques are called a martensitic metals—typically they are created by mixing two or more metals together to create an alloy and then applying heat to cause the metal to revert back to a desired state after being deformed.

Scientists have been working steadily for many years to create different metal alloys for different purposes—martensitic metals in particular are highly prized because they can be put to special purposes such as in creating stents for medical purposes.

But martensitic metals have one major drawback, they can only be disfigured and reversed a few times before they start to degrade, making them unsuitable for further use.

In this new effort, the research team in Minnesota has found a way to create one alloy that is able to withstand thousands of heating-cooling cycles without losing its reversibility attributes.

Their success the team explains, came about as a result of applying known mathematical theories related to stress properties of metals.

Martensitic metals degrade because of changes to a transition layer between the different phases.

Working the formulas, they report allowed them to eliminate this layer, effectively removing the part of the alloy that allowed for degradation.

To test their idea, the team chose a metal alloy that had previously been found to be very robust when exposed to repeated thermal cycles.

They then fine-tuned the proportions of the base metal ingredients to conform with what they had worked out with the math formulas until they hit on one that was nearly impervious to repeated thermal cycles.

The team reports that the alloy they developed also had an unusual riverine microstructure not seen before in a martensitic metal.

Perhaps more importantly, it appears that their technique might work with other alloys or non-metal materials, allowing for the creation of as yet unimagined real-world materials in a variety of applications.

More information: Enhanced reversibility and unusual microstructure of a phase-transforming material, Nature 502, 85–88 (03 October 2013) DOI: 10.1038/nature12532

ESO VLT: New technique boosts Detection of water on Exoplanets

An artist’s impression of a hot Jupiter (at bottom right), a giant planet that orbits extremely close in to its host star. 

Credit: Leiden Observatory

Using ESO's Very Large Telescope (VLT), a team of astronomers have been able to detect the tell-tale spectral fingerprint of water molecules in the atmosphere of a planet in orbit around another star.

The discovery endorses a new technique that will let astronomers efficiently search for water on hundreds of worlds without the need for space-based telescopes.

Dr Jayne Birkby of Leiden University will present the new result on Friday 5 July at the RAS National Astronomy Meeting in St Andrews, Scotland.

Jayne Birkby

Since the early 1990s scientists have found almost 1000 planets in orbit around other stars.

These so-called exoplanets are mostly much larger than the Earth and many are much closer to their stars than we are to the Sun, leading them to be described as 'hot Jupiters'.

In the new work the team studied the exoplanet HD 189733b, a world that orbits its star every 2.2 days and is heated to a temperature of over 1500 degrees Celsius.

Astronomers usually find exoplanets by measuring the gravitational influence of the planet on the star, which acts to pull the star around in a very small orbit, at velocities of a few kilometres per hour.

This movement causes a small shift in the lines of the stellar spectrum (known as the Doppler shift), which move back and forth with the wobble of the star.

The Leiden University-led team have flipped the technique on its head by measuring the gravitational influence of the star on the planet, which is much larger, hurling the planet around its orbit at some 400,000 km per hour.

They measured this by tracing the Doppler shift of the water lines in the exoplanet's spectrum as it orbited the star.

Despite the much larger velocity of the planet, it is nearly a thousand times fainter than the star, which makes detecting it very difficult.

The team were able to detect the spectral line of water in the exoplanet atmosphere by using the CRyogenic high-resolution InfraRed Echelle Spectrograph (CRIRES) instrument mounted on the VLT.

More information: The new work appears in "Detection of water absorption in the dayside atmosphere of HD 189733 b using ground-based high-resolution spectroscopy at 3.2 microns", J. L. Birkby, R. J. de Kok, M. Brogi, E. J. W. de Mooij, H. Schwarz, S. Albrecht, I. A. G. Snellen, submitted to Monthly Notices of the Royal Astronomical Society. A copy of the paper can be viewed at arxiv.org/abs/1307.1133