by Juan Apolinario C. Reyes
Two days ago, I saw the news about new exoplanets discovered by NASA. They didn’t just found one, they found seven. All occupying the same habitable zone, all revolving around the same star. This collection of space objects forms a solar system pretty much like our own solar system. Astronomers called it TRAPPIST -1.
It’s nice to see that science news is occasionally given front page publicity in news papers. But I was more than excited to read it because it is by profound coincidence that we just finished a chapter on Physical Science where we discussed our solar system, exoplanets, the methods to discover them, habitable zones, Doppler shift, etc. The stuff of topics discussed in an introductory course to astronomy.
HABITABLE ZONE
In popular science, this is what is called the goldilocks zone, an orbital plane around a star where planets receive the right quantity of radiant energy to allow water to stay in liquid form. For our solar system, that happens to be a zone which is 150 million km away from the center of the sun. But distance is not the ruler to use to find habitable zones, it is the quantity of radiant energy emitted by the star in a particular solar system which is.
DOPPLER SHIFT AND METHOD OF TRANSIT
How were exoplanets discovered at the first place?
The two most common methods astronomers use to find exoplanets are Doppler shift and the method of transit.
Planets revolving around a star exert a gravitational force on that star in the same way that Earth, too, exerts force on our sun. This force of gravitational attraction can be given a quantity using Newton’s law of universal gravitation. The combined gravitational force of planets on a star pushes and pulls that star in such a way that the star acquires a wobble. As planets revolve around a star, the star, in turn, moves in an orbit of its own. Stars of solar systems are never stationary in one place. They are locked in a circular dance with planets.
This wobble produces a shifting in the light spectra emitted by the star as it is observed from our planet. When the star moves away from Earth, its light is redshifted. When it moves towards Earth, it is blueshifted. The cycle of blue-red shifting in the star’s light spectra is an evidence of mass of a nearby space object. It could be the mass of another star, or maybe the mass of an exoplanet.
The second method is through transit. An object is in transit when it is in a state of movement from one point to another. With exoplanets, we are referring to the movement of a planet of a distant star system as the planet crosses the field of vision between its star and Earth. In actuality, planets in transit are rarely visible because the glare of the star behind them overwhelms their image. But during transit, their sun displays a dip in luminosity. A planet in transit blocks a small quantity of the star’s radiant energy and this is manifested as a dip in its brightness. When astronomers see a cyclic dip in luminosity from a star, this could be an indication of an exoplanet in transit.