We stand on a great threshold in the human history of space exploration. On the one side of this threshold, we know with certainty that planets orbiting stars other than the Sun exist and are common… If life is prevalent in our neighborhood of the Galaxy, it is within our resources and technological reach to be the first generation in human history to finally cross this threshold, and to learn if there is life of any kind beyond Earth.
—Sara Seager, the Canadian-American astronomer, planetary scientist and professor at MIT, in her testimony before the United States House of Representatives Committee on Science, Space, and Technology in 2013.
But one day, many of us will gaze at the encyclopedia containing the coordinates of perhaps hundreds of earth-like planets in our sector of the galaxy. Then we will wonder, as Sagan did, what a civilization a millions years ahead of ours will look like…
— Michio Kaku, the American theoretical physicist and Henry Semat Professor of Theoretical Physics at the City College of New York, in the blog “The Physics of Extraterrestrial Civilizations. How advanced could they possibly be?”
If our Sun has planets, shouldn’t other stars have planets as well? Moreover, if one of those planets in our Solar system harbors life, can’t we expect to find life in some distant planet that revolves around a different star too?
The interest in discovering life forms elsewhere in of our Galaxy, or in the Universe for that matter, has received a huge boost recently with NASA’s new found ability to detect planets in Solar Systems other than ours. Twenty five years ago, nothing was known about any other worlds outside of our Solar System. Thanks mainly to NASA’s Kepler space telescope, which was launched in March 6, 2009 to explore our galaxy’s interplanetary systems, we now know the Milky Way is teeming with planets, some of them resembling our own Earth.
A planet that does not orbit our Sun but orbits a different star, or a brown dwarf is called an exoplanet. According to Wikipedia more than 1800 exoplanets have been discovered (1885 planets in 1184 planetary systems including 477 multiple planetary systems) as of 27 January 2015. Almost all of these reside inside our Milky Way. But a few of them could be of extragalactic origin.
Extrasolar planets are indirectly detected by observing the effects that they have on the parent star.
The first extrasolar planet was discovered in 1994, by Alexander Wolszczan, a radio astronomer at Pennsylvania State University. Wolszczan had discovered two or three planet-sized objects orbiting a pulsar in the Virgo constellation. He observed regular variations in the pulsar’s rapidly pulsated radio signal that indicated the planets’ complex gravitational effects on the dead star. It was quickly realized that these worlds were barren and inhospitable as they are permanently bathed in high-energy radiation from the pulsar.
In 1995, two Swiss astronomers, Michel Mayor and Didier Queloz of Geneva found a rapidly orbiting planet located close to the star 51 Pegasia, a star similar to our sun. The planet was observed indirectly using the radial velocity method. The mass was determined to be between half to two times the mass of Jupiter.
These announcements marked the beginning of a flood of discoveries. Three months later, a team led by Geoffrey W. Marcy and Paul Butler of San Francisco State University and the University of California at Berkeley confirmed the discovery and added two more planets to one discovered by the Swiss team. By the end of the 20th century, several dozen exoplanets were discovered by carefully observing nearby stars for several months or even years.
Today we know, most of the exoplanets found so far are enormous gas giants such as the Jupiter or Saturn in our own Solar System. But most mind boggling is their variety. Well known planetary scientist and astrobiologist Sara Seager: “Some stars have a giant planet like Jupiter where the Earth would be. Other stars have planets like Jupiter 10 times closer to them than Mercury is to our Sun. Some stars have planets we call ‘super-Earths”, rocky worlds bigger than Earth but smaller than Neptune”. A handful of them are believed to be Earth-sized planets (their numbers are growing rapidly).
The obvious question now is: what are the chances these exoplanets could harbor life?
The massive gaseous stars are not suitable for life because, in general, they have no surface and are too hot for life. One of the main ingredients for life, as we know it, is liquid water. Water can remain in a liquid state between 273K and 373K (unless the pressure is too low, in which case the water turns into vapor). The region in a planetary system where the temperature is in this range, is called the habitable zone, or HZ. See Figure 11-1 for a visualization of a habitable zone relative to a star. The red region is too warm, the blue region too cold, and the green region is just right for liquid water. However, there is another important consideration too. Most stars that are being searched for life-bearing planets must survive long enough for its planets to develop life.
The discovery of habitable exoplanets is of particular interest, especially Earth-like rocky worlds which are the best candidates to start looking for signs of life. To be Earth-like, a planet must be both Earth-sized (less than 1.25 times Earth’s girth and less than twice Earth’s mass) and must circle its host star inside the habitable zone. Table 1 shows a list of potentially habitable Earth-like planets identified as of January, 2015, branded based on the above criteria. Public interest piques every time the discovery of an Earth-like exoplanet is announced. However, the current detection techniques cannot say if the planet is truly Earth-like. For that, we ultimately need to get a spectrum, an impression of the planet’s atmosphere, to find out whether the planet is indeed habitable to life.
|Table 1: Number of Potentially Habitable Exoplanets as of January, 2015.|
By the next decade scientists expect to use the “Direct Imaging” method to measure an Earth-like planet’s atmosphere. This is the only planet discovery technique that is capable of separating the planet’s reflected light from its star’s much brighter glare. For comparison consider this: the Earth is 10 billion times fainter than the Sun at visible wavelengths. What would it take for an alien civilization in another Solar System to take an image of the Earth so that they can perform a spectral analysis of the Earth’s reflected light to find biosignature gasses such as oxygen, carbon dioxide, methane, traces of ozone, and water vapor in our atmosphere? They would surely need very powerful telescopes in space with special devices to filter out Earth’s reflected light from the Sun’s much brighter shine.
To make the Direct Imaging method economically feasible, scientists have proposed to align the James Webb Space Telescope (JWST), which is scheduled to launch in 2018, with a specially shaped giant screen called an “external ocular” to capture just the planet’s reflected light by blocking its star’s light from reaching the telescope’s mirror. The ocular will be deployed 30,000 miles away from JWST and perfectly aligned with it, a difficult but very much achievable proposition.
Today, NASA’s Kepler space telescope has provided a critical tally of exoplanets. It has also found a multitude of small, Earth-like exoplanets. But those are too distant from the Earth for any realistic studies of their atmospheres. In the near term, NASA has plans for a new satellite mission called Transiting Exoplanet Survey Satellite (TESS) to be launched in orbit around 2017 that will be capable of finding Earth-size and super Earth-size exoplanets (up to 1.75 times Earth’s size) transiting M stars, stars which are significantly smaller, cooler, and more common than our Sun. The TESS-JWST combination is eventually expected to reveal information on the birth of galaxies, locate (possibly) habitable worlds orbiting other stars, and track asteroids that might impact Earth.
To quote Sara Seager:
“Someday, sooner or later, we will know of bright stars that host living planets very much like Earth. We will be able to stand beneath a dark sky and point out to our friends or family “That star has a planet like Earth.” This is a fantastic time for exoplanets, and for astronomy. But the future will be better still.”
12. EXOPLANETS AND SETI
It should be clear by now the odds of accidentally stumbling into an alien radio signal is extremely low. It therefore helps to know where to look. Exoplanets, particularly Earth-like rocky worlds are the most obvious places to look. Unsurprisingly, astronomers have already started to use data from the Kepler space telescope for a more targeted “listening” of radio signals coming from stars known to have planets orbiting them.
The first exoplanet discovered by Kepler is located 180 light-years away in the constellation Pisces. Named HIP 116454b, the exoplanet measures 2.5 times the diameter of Earth (and is therefore dubbed a ‘super-Earth’). HIP 116454b is by no means a habitable planet, with a rapid 9-day orbit around its host star. Due to its proximity to its host, HIP 116454b is probably tidally locked—the huge gravitational force from the host star would bind HIP 116454b into a tidally locked state with one of its hemisphere facing the host star, just like the Moon does to the Earth. Harmful ultraviolet radiation and the huge flares of energy from the nearby star would render HIP 116454b completely inhabitable.
Nevertheless, SETI has pointed the Allen Telescope Array (ATA), located in at Hat Creek Radio Observatory, 290 miles northeast of San Francisco, California, to study HIP 116454b.
But why would SETI bother aiming ATA at an unlikely target?
With sufficient future funding from its donors, SETI’s aim is to examine each planetary systems found by Kepler, although the highest priority will be to focus on the exoplanets inside their host stars’ habitable zones.
So far, the ATA has been looking for signals in the 1000—2250 MHz range emanating from HIP 116454b and soon higher frequencies will also be analyzed. ATA’s 42 radio antennae allow many frequencies to be monitored concurrently. If intelligent extraterrestrials have evolved to use similar technologies as we do, then perhaps one day we will be able to listen into their transmissions as well. Unfortunately, nothing untill now indicates the world is transmitting back to us, and therefore, this unluckily, is another case of a fruitless search.