Detection of Brown Dwarfs and Exoplanets with the WISE Mission

Since its launch in 2009, NASA’s Wide-Field Infrared Survey Explorer (WISE) mission has made invaluable contributions to the field of astronomy, providing us with vast knowledge about the universe, our Milky Way, and the solar system.

Image-1. Artist’s impression of the WISE mission telescope. (Credits: NASA/JPL-Caltech/UCLA)

In order to create a map of the sky, a 40cm diameter telescope placed in Earth orbit was used to study the coldest objects in the universe.

The telescope is equipped with detectors that are kept very cold at just 15 degrees above absolute zero (-430°F/15°K) with the help of a cryostat.

Image-2. Telescope photograph from the WISE mission, shown here without the aperture cover. The main mirror of the telescope is located at the end of the open tube. (Credits: NASA/JPL-Caltech/UCLA)

The WISE telescope has an infrared-sensitive digital camera that takes a photo every 11 seconds, each image covers an area of the sky 3 times larger than the full moon, in the first 6 months of the mission almost 1.500.000 will be acquired that you photograph everything covers the sky.

All data obtained in the WISE mission is downloaded by radio transmission 4 times a day for further processing.

Why does the WISE mission study the sky in the infrared wavelength?

Infrared observations are of great importance in the study of low-temperature media, there are objects in the universe such as interstellar dust grains, giant exoplanets, brown dwarfs, as well as the icy surfaces of planetary satellites and asteroids that have a temperature that varies between 3 to 3000°K, the energy radiated by these objects is within the temperature range in which the infrared wavelength is found.

Image-3. 2-D map of the sky observed by the WISE mission in four bands W1, W2, W3 and W4 centered at 3.4, 4.6, 12 and 22 m respectively, in the mid-infrared wavelength. (Credits: NASA/JPL-Caltech/UCLA).

In such a way that the observations in this wavelength serve to study those regions “hidden” in space by cosmic dust, for example star formation regions.

Another advantage of infrared observations is that they allow us to study the young universe due to the expansion of the Universe, this through the redshift or Doppler Effect, the further an object moves away, its light will shift to longer wavelengths proportionally, since the speed of light is finite, objects with a greater redshift will be observed as they were in the young universe.

Most of the radiation emitted by very distant objects such as stars, galaxies and quasars is now in the infrared.

Image-4. Collection of galaxies of different types obtained by the WISE mission. (Credits: NASA/JPL-Caltech).

The WISE mission and the detection of brown dwarfs

Image-5. Artist’s impression of a young brown dwarf named WISEA J114724.10-204021.3. (Credits: NASA/JPL-Caltech/UCLA).

One of the main goals of the WISE mission is the detection of brown dwarfs, truly amazing substellar objects, they have very low mass so they cannot fuse Hydrogen into Helium like main sequence stars, their mass range (75-80 Jovian masses) is among the lightest stars and heavy gas giant planets like Jupiter, over time these gradually fade and cool until reaching equilibrium, the oldest brown dwarfs that exist in the universe are very cold and weak for example WISE J085510.83-071442.5 discovered in 2014, it is only 7.2 light years away, within our cosmic neighborhood it is one of the closest to the sun, it has a temperature that ranges between 225 and 260°K and a mass between 3 and 10 times that of Jupiter.

Image-6. The diagram shows the location of the closest stars and brown dwarfs to the Sun. (Credits: NASA/Penn State University).

Detecting brown dwarfs directly is very difficult as they hardly emit light even those closest to the sun, however they have a very strong emission at wavelengths of 4.6 m due to lack of methane absorption, therefore, WISE is a powerful tool for finding really cool brown dwarfs.

Lithium is an element that allows us to identify brown dwarfs through their characteristic emission spectra, in a brown dwarf hydrogen does not reach the temperature and pressure necessary to trigger nuclear fusion and therefore lithium is not destroyed, remaining in it throughout its existence.

So this is the classical method for its identification, although a disadvantage is that in some observations of low-mass stars lithium can be detected because it had not yet been consumed due to the slow reactions in such stars.

Image-7. Emission spectrum of lithium. (Credits: Casanchi.org).

Brown dwarfs, like stars, are classified by spectral classes, these classes are of type M, L, T and Y, depending on their temperature, with type Y being the coldest.

Brown dwarfs change their spectral type as they age because they have no other source of energy, they cool down to type Y.

If we could observe brown dwarfs directly, we would see a variety of colors ranging from magenta to red or orange, contrary to what its name indicates.

Image-8. Comparison of the different types of brown dwarfs, low-mass stars, and planets like Jupiter. (Credits: MPIA/ V. Joergens).

Theoretically, the population density of brown dwarfs is thought to be about twice the density of ordinary stars in the universe. In total, the WISE mission has detected 200 brown dwarfs to date, 13 of these are of the Y type.

Brown dwarfs and exoplanet detection

As we have already seen, brown dwarfs are found between the limits of low mass dwarf stars and gas giant planets, to distinguish them the observation of Deuterium nuclei is used, the planets are objects that never reach the necessary temperature to destroy deuterium (1.2 million Kelvin) and have a mass less than 14 Jupiter masses, brown dwarfs develop degenerate interiors, which prevent the core temperature from reaching the stable melting point of Hydrogen, but do destroy Deuterium.

Image-9. Illustration of a planet that is four times the mass of Jupiter and orbits 5 billion miles from a companion brown dwarf. (Credits: NASA/JPL-Caltech/UCLA).

Currently more than 5000 exoplanets have been discovered, the first discovery of an exoplanet orbiting a brown dwarf was ChaHα8, it has a mass ratio with its primary companion of approximately 0.3, it is so close that it has been suggested that it is actually a binary system.

In 2013, OGLE-2012-BLG-0358Lb was discovered, it was the first planetary-mass companion in a relatively small orbit around a brown dwarf.

Later in 2015, the first Earth-mass planet OGLE-2013-BLG-0723LBb was found orbiting a brown dwarf.

These discoveries laid the foundation for the detection of exoplanets by observing brown dwarfs.

Another great discovery was the discovery of accretion disks for planet formation around brown dwarfs, with characteristics similar to those found in stars, most of the observed disks have very small masses, so they correspond to the formation of Earth-sized planets instead of gas giants.

During the early stages of planet formation, the micron-sized dust grains accumulate into larger and larger particles, the reduction in the area occupied by the dust grains causes the disk to become thinner, at the same time shrinking. its emission in the infrared spectrum.

The disc does not lighten uniformly due to this, a lack of uniformity in the distribution of the infrared as a function of wavelength can be observed. WISE can detect these signatures to define the formation of the planetary system.

Image-10. Artist’s impression of a brown dwarf surrounded by a spinning disk of planet-forming dust. (Credits: NASA/JPL-Caltech).

The possible habitability of planets orbiting a brown dwarf has been studied. The data obtained show that the conditions for a brown dwarf to have a habitable planet are extremely strict because these stars cool over time and also because the eccentricity orbital (the shape of their orbit) of such planets must be extremely low to avoid creating tidal forces that eventually produce an uncontrolled greenhouse effect on them that makes them uninhabitable.

Final phase of the WISE mission

The WISE mission was launched in December 2009 and put into hibernation on February 17, 2011, because the initial mission’s duration was limited by its hydrogen coolant.

In September 2013 WISE was reactivated and was renamed NEOWISE with a new goal: to identify and characterize the population of Near-Earth Objects, potentially hazardous asteroids and comets, to provide more information about their sizes and compositions.

Some of the objects observed by the NEOWISE mission have been classified as potentially hazardous (PHA), based on their size and how close they can get to Earth’s orbit.

Image-11. Review of the number of near-Earth asteroids by WISE. (Credits: NASA/JPL-Caltech/UCLA)

NEOWISE collected more than 2.5 million infrared images of the sky in its seventh year of operations. This data is combined with the first six years of NEOWISE data into a single publicly available archive, containing approximately 17.8 million image sets and a database of more than 133 billion source detections drawn from those images.

Also, thanks to the NEOWISE project, a large number of minor planets have been discovered. To date, the mission has detected approximately 158.000 asteroids, of these 1.850 are near-Earth, and 300 comets have also been detected. All these data obtained allow a variety of studies on the origins and evolution of small bodies in our solar system.

Image-12. WiSE. All objects (asteroids and comets) found by the NEOWISE mission during the first 8 years of reactivation. (Credits: NASA/JPL-Caltech)

The WISE mission catalog and the atlas of images obtained are very valuable and have been used for a large amount of research in the field of astrophysics, from asteroids in the solar system to the most luminous distant galaxies in the visible universe.

Currently, data from the WISE mission continues to be used by various science teams and in citizen science projects where even more objects are expected to be discovered.