Galaxy is good at making planets
Nasa’s top scientist on the Kepler project says planets are far more diverse than previously understood. Will Harvie reports.
One of the first exoplanets was identified in 1995 and it set off a gold rush of science to find other planets outside our solar system and within the Milky Way.
In the years that followed, 326 exoplanets were discovered, mostly by ground-based technologies. Then, in March 2009, the US National Aeronautics and Space Administration (Nasa) launched the Kepler space observatory satellite.
In its first four years, Kepler helped identify 3483 exoplanets, 2482 of them confirmed with 99 per cent accuracy and the balance running at about 90 per cent accuracy.
Kepler gave scientists a ‘‘kick in the pants’’, said Nasa scientist Dr Natalie Batalha while on a speaking tour of New Zealand last week.
The Milky Way makes planets with remarkable regularity, she said.
‘‘When you look up into the sky ... all of those pinpoints of light aren’t just stars, they’re planetary systems – just about all of them,’’ she said. ‘‘Planets are very common.’’
Batalha was the project scientist for the US$600 million Kepler mission. Its primary mission was to find Earth-like planets in the Milky Way that might harbour life.
Others flew the spacecraft, so to speak, and until 2015 she led the interpretation of the data that came in from the 95-centimetre telescope aboard the craft.
Among the findings was that ‘‘nature makes small planets pretty efficiently’’, she said. Large planets are rarer.
This was a revelation because the pre-Kepler exoplanet discoveries were typically large, Jupiter-sized planets that orbited close to their suns.
The Kepler mission was to find planets capable of harbouring life. Batalha and colleagues sought small planets, roughly bigger than Earth but smaller than Neptune.
They were looking for planets that circle G-type stars – the type most like our sun and which behave themselves.
They also sought planets that are not too hot and not too cold. This put them in the ‘‘habitable zone’’ – not so close that energy burns off elements, especially liquid water, but not so far away that they are frozen.
About 50 of these ‘‘Goldilocks’’ planets have been identified from the data analysed so far, Batalha said.
One of the most promising is Kepler-452b. It’s thought to be about 60 per cent larger than Earth but orbits a G-type star that’s about 10 per cent larger than our sun. One orbit takes about 385 days. It receives about the same amount of energy as Earth and might be rocky.
Another trend emerging from the Kepler data shows that the diversity of planets in the Milky Way far exceeds the diversity of planets in our solar system, Batalha said.
Nasa has found planets orbiting exotic stars – pulsars and white dwarfs for example. They’ve found planets orbiting up to 30 times closer to their star than Mercury does to the sun. They’ve been dubbed ‘‘lava worlds’’ because their surfaces must be molten.
Some planets are so close to their star that they are disintegrating and leave a cometlike tail of debris that Kepler can detect.
Other exoplanets orbit two stars and some obit within star clusters, dense conglomerations of many stars. ‘‘If you lived on one of these planets, you’d look up to ... a bejeweled sky,’’ she said.
Speaking of a particular class of exoplanets, Batalha said: ‘‘We don’t know much about them yet’’. She could have been speaking about all of them.
That’s in part due to the technique used by Kepler scientists to detect exoplanets. It’s very much like the technique Captain James Cook used to observe the 1769 transit of Venus. He had to sail to the South Pacific (and later visited Aotearoa) to observe Venus crossing the face of the sun.
Kepler scientists can’t physically see an exoplanet transiting a distant star. Instead they observe a faint dimming of light that occurs as an exoplanet crosses the face of the star and blocks some radiation.
In simple terms, the amount of dimming tells scientists the size of the planet, Batalha said, while the frequency of dimming explains a planet’s distance from its star.
To grasp the amount of dimming, Batalha asked an audience to consider an 18-storey Auckland skyscraper. It’s night and every light is turned on and every blind open. Then somebody lowers one blind 2 centimetres. That’s the equivalent of energy dimming that Kepler detects – from light years away.
And even then, data is only available when planets cross the line of sight between the star and Kepler. Some stars are obscured or only visible from a particular place. This is why Cook had to travel to the South Pacific to see the transit of Venus and couldn’t simply watch from the royal observatory at Greenwich.
Batalha acknowledged that Nasa limited itself to looking for planets capable of hosting carbonbased life that relies on liquid water. It’s entirely possible that life exists in an ammonia environment like that on a moon of Saturn.
But given carbon and water fostered life on Earth, it’s reasonable to think they might do the same elsewhere.
Nasa plans other missions that will follow Kepler. The James Webb space telescope will launch in early 2019 and use an infrared telescope with a 6.5-metre primary mirror to look at exoplanets (among other missions).
Meanwhile, the Transiting Exoplanet Survey Satellite (Tess) will survey 200,000 of the brightest stars near the sun to search for transiting exoplanets.
Both missions are expected to greatly increase the number of known exoplanets and determine the composition of atmospheres. They’ll be looking for the byproducts of life.
Batalha finished her talk with an intriguing thought. It’s believed about 200 stars with exoplanets have good line of sight of Earth. It’s possible intelligent life on one of them is peering at Earth and wondering if life exists here.