Staring at the Sky

NASA satellite finds new exoplanets just months after launch

Many of us wonder what lies beyond Earth and our solar system. Star Wars, The Martian, and other science fiction works have helped to quench our imaginations, but perhaps it need not all be fiction. NASA’s recently retired Kepler satellite lasted approximately one decade on a mission to discover Earth-sized planets in habitable regions of stars. The Kepler satellite looked at one part of the Milky Way for years, whereas a promising new satellite is currently exploring the entire sky utilizing similar technology. NASA launched the Transiting Exoplanet Survey Satellite (TESS) in April 2018 for a two-year mission. In its first four months of surveying, it has already successfully identified eight exoplanets, planets that lie beyond our solar system, and observed more than three hundred others for follow-ups. These are of great interest to scientists, as the data collected by satellites like Kepler and TESS can help answer questions about life in the universe and the suitability of other planets to sustain life.

Transiting—the science behind satellites

Kepler and TESS are able to determine the presence of exoplanets by monitoring the brightness of a particular star over time and detecting dips in brightness due to transiting, the movement of a planet in front of the star. A planetary transit reduces the brightness of a star as it passes across. The planet’s orbit can be deduced from the time between dips in brightness.

TESS is expected to be a more effective planet hunter than Kepler, because TESS surveys a new patch of sky each month. It will take just two years for TESS to scan all 360 degrees of sky seen from Earth. Since TESS spends a short amount of time scanning each sky segment, scientists do not expect it to find many planets with periods, the time it takes for a planet to make one full orbit, longer than one Earth month. Longer-period planets transit less frequently; therefore, TESS is expected to primarily detect planets with periods less than ten Earth days.

Nevertheless, TESS has already proudly confirmed the existence of HD 21749b, an exoplanet with a period of 35.61 Earth days. The density of HD 21749b suggests a rather gaseous atmosphere. “The density is really valuable. It gives a sense of the composition of the planet,” said Diana Dragomir, postdoctoral researcher at the MIT Kavli Institute for Astrophysics and Space Research and member of the TESS team. Calculations like density, mass, temperature, and planet size are made possible by TESS technology, specifically through measuring how big of a dip in brightness occurs during transit. Furthermore, the magnitude of the dimming is related to the planet’s mass and diameter relative to the star’s size. The star’s size can be determined using asteroseismological data from TESS. Additionally, the orbital size of the planet and the temperature of the star can be used to calculate the temperature of the planet. Evidently, planets cannot be considered isolated bodies, but part of a larger celestial system, particularly dependent on the stars they orbit.

TESS and the life question

TESS is about the size of a big fridge and has four telescopes, each about ten centimeters in diameter, rather small compared to the Hubble Telescope. “It has a pretty funky orbital; it approaches Earth once per orbit and then swings out to avoid light reflected from Earth on the camera,” Dragomir said. Such light from Earth would be considered as contaminating measurements of light from distant stars. “We are looking in the inhabitable zone of stars for planets able to store water,” Dragomir explained. Earth is the perfect distance from the sun, making it able to hold water and sustain the immense diversity of life that it does. TESS attempts to find exoplanets that, like Earth, are located in what is known as the Goldilocks zone. “We think the results will answer the life question,” Dragomir said.  

Outlandish new worlds

Of the over three hundred exoplanets that TESS has observed in its first four segments of the galaxy, many are already wondering what remains to be found. One strange finding was Pi Mensae c, reported in September 2018. Pi Mensae c has a 6.27-day orbit and is 2.14 times the diameter of Earth, with 4.8 times Earth’s mass, such that its density is close to that of water. Another planet, Pi Mensae b, orbits the same star but much more slowly, with a period of 5.7 years, and has 3170 times Earth’s mass. It is rather unusual that Pi Mensae c is able to survive the eccentric orbital swinging caused by Pi Mensae b.

TESS has also discovered a planet covered in lava known as LHS 3844b. It is 1.3 times the size of Earth but maintains an orbit of eleven Earth days. Its surface temperature of 540 degrees Celsius makes it interesting–but unfortunately, not a viable alternative to Earth for human life.

The TESS mission goes beyond simply finding exoplanets. The information collected by TESS is continuing the space revolution with more rigor than any satellite before. “We will have the means to detect things like oxygen on other planets,” Dragomir said. Every detail discovered about other planets will help scientists understand Earth more thoroughly, as well as help improve TESS to plan future missions. Processes like planet formation can be more clearly understood by collecting a large amount of data from TESS’s search. For instance, planetary composition is key information that can help researchers discern the chemical reactions able to take place on individual planets–some of which are essential for sustaining life. 

Infinite promise

 “We’re working to extend the mission from two years,” Dragomir said. She believes that TESS will be in space for a while. In an effort to make full use of the TESS mission, NASA plans to launch the James Webb Space Telescope in 2020 to follow up on some of TESS’s most prominent discoveries. Besides directly hunting for planets and analyzing their physical characteristics, TESS is able to indirectly learn about planets through asteroseismology, a technique where stellar sound wave vibrations in stars are used to determine the composition of stars, their ages, and their sizes. Such sound waves result from temperature changes inside the star’s convection zone, an area between the core and visible surface where hot plasma rises, cools, and falls on repeat. When the plasma falls toward the core, it releases an energy wave that makes the star expand and contract, essentially making the star comparable to a bell. This bell rings through space. TESS is capable of sensing the star sounds while in space by seeking changes in star brightness, which is indicative of star “ringing.” Knowing more about stars can do much to shed light on the planets that orbit them.

TESS has shown incredible promise in a rather short time frame. The conclusions of the mission are impossible to predict but exciting to imagine. TESS has proven that it is capable of turning science fiction into reality.