July 2015: The Kepler Space Telescope made an extraordinary exoplanet discovery, Kepler-452b
Kepler-452b, in the constellation Cygnus, is 1400 light years away shares the same spectral classification as our Sun which is a G2 star.
NASA Kepler News Page reported:
“NASA’s Kepler team announced discovery of planet Kepler-452b which has the closest match of planet characteristics to Earth: 60% larger than Earth, orbiting a Sun-like star in a 385 day orbit period, that puts it in the star’s habitable zone”
This makes us wonder…
- How is the planet detected?
- How is the orbital period determined?
- What makes the Star ‘Sun-like’?
To Detect Exoplanets – Astronomers use Transit Photometry
Measuring the tiniest dips in brightness on the graph of the radiation output of the stellar system is the key to astronomical Transit Photometry: We measure the light emitted and graph the brightness on a light curve (green graph below) , looking for tell-tale variations. These variations may mean there are 2 bodies orbiting each other, or for some other reason there are changes in the output radiation when graphed over time.
Whenever a planet passes in front of its parent star as viewed from the spacecraft, a tiny pulse or beat is produced. From the repeated beats we can detect and verify the existence of Earth-size planets and learn about the orbit and size of the planet. As it orbits its star, we are lucky to see that orbit edge-on. Once every orbit, the planet passes directly in front of the star and we see a tiny dip in starlight
- .The size of that dip correlates to an estimate of the size of the planet, and the time it takes tells us the planet’s orbital period. The dimming of Kepler 452B during transit directly reflects the size ratio between the star and the planet: A small planet transiting a large star will create only a slight dimming, while a large planet transiting a small star will have a more noticeable effect.
- The size and mass of the host star can be known with considerable accuracy from its spectrum (which is happily G2 – like our Sun), and photometry therefore gives astronomers a good estimate of the orbiting planet’s size.
The NASA Kepler page walks us through the steps of detecting a simulated Star/Planet system with The Kepler Exoplanet Transit Hunt
Pick a Star – with stellar classification of G2 (to be a sun-like star):
Next we measure the dip in brightness which takes several hours but over the course of a year represents a blink in time.
- Notice the measured time in days for the transit.
- Notice too the very tiny dip in brightness. No terrestrial detector could be sensitive enough to measure .027% dip in brightness!
Get the distance from the Star by knowing the Orbital Period
We can apply Kepler’s Third Law of Planetary Motion:
- For our candidate Star – the Mass is similar to 1 Solar Mass.
- We discover that the distance is similar to the Earth-Distance unit of 1 Astronomical Unit, the distance from the Earth to the Sun. However, that distance varies as the Earth orbits the Sun, from a maximum (aphelion) to a minimum (perihelion) ) The astronomical unit is used primarily as a convenient yardstick for measuring distances within the Solar System or around other stars.
- Imagine! We are detecting such small distances when we realize this stellar system is 1400 light years away! ( A light year is the distance light travels in a year: 9 trillion kilometers (9 thousand billion kilometers))
One major property that make this planet special is the sun-like classification of its stellar host . The NASA Kepler Mission is capable of searching for stars that match the spectral components of our Sun. We learn in the Night Sky Course about the importance of the analysis of starlight and that Spectra are the fingerprints of the Stars ..Stars and Starlight in our Milky Way
The Kepler Spacecraft is constantly monitoring stars in a very dense portion of the Milky Way in the Constellation Cygnus. This is a summer Constellation for Northern Hemisphere observers. The stellar magnitude of these stars is too faint to observe them the way that Kepler does, but it is in the portion of the Milky Way that hosts other extraordinary celestial treats…
However, we will have to wait until next month when the Night Sky temperatures dip below at least 15 degrees so that we can sit outside and comfortably observe!
… Else we look like this…
Or perhaps, if we fashionably dress for a starparty .. like this: