Faculty of Science Dean Ray Jayawardhana
For the first time, a team of astronomers – including York University physics and astronomy Professor and Faculty of Science Dean Ray Jayawardhana – have measured the passing of a super-Earth in front of a bright, nearby, sun-like star using a ground-based telescope. The transit of the exoplanet 55 Cancri e is the shallowest detected from the ground yet, and the success bodes well for characterizing the many small planets that upcoming space missions are expected to discover in the next few years.
The international research team used the 2.5-metre Nordic Optical Telescope on the island of La Palma, Spain – a moderate-sized facility by today’s standards – to make the detection. Previous observations of this planet transit had to rely on space-borne telescopes.
During its transit, the planet crosses its host star, 55 Cancri, located just 40 light years away from us and visible to the naked eye, which blocks a tiny fraction of the starlight and dims the star by 1/2000th (or 0.05 per cent) for almost two hours.
“Our observations show that we can detect the transits of small planets around sun-like stars using ground-based telescopes,” says Professor Ernst de Mooij of Queen’s University Belfast, U.K., the study’s lead author. “This is especially important because upcoming space missions such as TESS and PLATO should find many small planets around bright stars.”
TESS is a NASA mission scheduled for launch in 2017, while PLATO is to be launched in 2024 by the European Space Agency; both will search for transiting terrestrial planets around nearby bright stars.
This artist’s concept contrasts our familiar Earth with the exceptionally strange planet known as 55 Cancri e. While it is only about twice the size of the Earth, NASA's Spitzer Space Telescope has gathered surprising new details about this supersized and superheated world.
Astronomers first discovered 55 Cancri e in 2004, and continued investigation of the exoplanet has shown it to be a truly bizarre place. The world revolves around its sun-like star in the shortest time period of all known exoplanets – just 17 hours and 40 minutes. (In other words, a year on 55 Cancri e lasts less than 18 hours.) The exoplanet orbits about 26 times closer to its star than Mercury, the most Sun-kissed planet in our solar system. Such proximity means that 55 Cancri e's surface roasts at a minimum of 3,200 degrees Fahrenheit (1,760 degrees Celsius).
The new observations with Spitzer reveal 55 Cancri e to have a mass 7.8 times and a radius just over twice that of Earth. Those properties place 55 Cancri e in the "super-Earth" class of exoplanets, a few dozen of which have been found. However, what makes this world so remarkable is the resulting low density derived from these measurements.
The Spitzer results suggest that about a fifth of the planet's mass must be made of light elements and compounds, including water. In the intense heat of 55 Cancri e's terribly close sun, those light materials would exist in a "supercritical" state, between that of a liquid and a gas, and might sizzle out of the planet's surface.
Only a handful of known super-Earths, however, cross the face of their stars as viewed from our vantage point in the cosmos. At just 40 light years away, 55 Cancri e stands as the smallest transiting super-Earth in our stellar neighborhood. In fact, 55 Cancri is so bright and close that it can be seen with the naked eye on a clear, dark night. Photo: NASA JPL
“It’s remarkable what we can do by pushing the limits of existing telescopes and instruments despite the complications posed by the Earth’s own turbulent atmosphere,” says Jayawardhana, the study’s co-author and de Mooij’s former postdoctoral supervisor. “Observations like these are paving the way as we strive towards searching for signs of life on alien planets from afar. Remote sensing across tens of light years isn’t easy, but it can be done with the right technique and a bit of ingenuity.”
The planet 55 Cancri e is about twice as big and eight times as massive as the Earth. With a period of 18 hours, it is the innermost of five planets in the system. Because of its proximity to the host star, the planet’s dayside temperature reaches over 1,700 Celsius – hot enough to melt metal – with conditions quite inhospitable to life. Initially identified a decade ago through radial velocity measurements, it was later confirmed through transit observations with MOST and Spitzer space telescopes.
Until now, the transits of only one other super-Earth, GJ 1,214b circling a red dwarf, had been observed with ground-based telescopes. The Earth’s roiling air makes such observations extremely difficult. But the team’s success with 55 Cancri e raises the prospects of characterizing dozens of super-Earths likely to be revealed by upcoming surveys.
Next, the team plans to search for steam (water) in the planet’s atmosphere.
The research team also includes Mercedes Lopez-Morales of the Harvard-Smithsonian Center for Astrophysics, as well as Raine Karjalainen and Marie Hrudkova of the Isaac Newton Group of Telescopes. Their findings will appear in the Astrophysical Journal Letters.