More Planets than Stars – But Axial Tilt is the Key to Life

There is an average of more than one planet per star in the Milky Way
Image Credit: NASA / ESA / ESO

With the forthcoming publication in the journal Nature on 12 January, it is estimated that there are more than 100 billion planets in our Milky Way galaxy. That means more than one planet per star, and results show that there are more rocky small Earth-like planets than giant Jupiter-size gas planets.

Most recent discoveries have come from the Kepler Observatory using transit observations. Some of the earliest confirmation of gas giants came from radial velocity Doppler observations.

The conclusions in the Nature article are based on micro-lensing studies.

Recent results from the Kepler Observatory have shown the existence of three small, rocky planets around the star KOI-961, a red dwarf. These three planets, named KOI-961.01, KOI-961.02 and KOI-961.03, are 0.78, 0.73 and 0.57 times the radius of Earth. The smallest is about the size of Mars (see below). Follow-up observations were made by the Palomar Observatory, near San Diego, and the Keck Observatory atop Mauna Kea in Hawaii.

Relative size of the three rocky planets around KOI-961
Image Credit: NASA / JPL-Caltech

Since it is now clear that rocky planets exist around millions, if not billions, of stars, the question arises as to whether there is life on them, and whether it may resemble life on Earth.

Whether a planet exists in the “Goldilocks” region around a star depends on many factors. Three factors include the type of star, how far away from the star the planet resides and the atmospheric pressure of the planet. A red dwarf, such as Gliese 581, means the planet has to be closer than the Earth to our Sun. A white hot star means the planet has to be farther away. And if the atmosphere is low, like Mars, or to high, like Venus, liquid water is not likely.

A fourth factor is axial tilt. If a planet has no axial tilt (the spin axis is perpendicular to the plane of its orbit around the star) then the polar regions freeze and the equatorial regions bake. There is little exchange between these regions due to atmospheric circulation. Axial tilt, such as the Earth has, allows distribution of heat between the equator and the poles.

Even if a planet has axial tilt, a recent study shows that interaction at a close distance (within the “Goldilocks” region) with red dwarf will eliminate axial tilt in less than 100 million years. Bacteria on Earth required 1,000 million years to evolve. Theoretically, a planet with no axial tilt could possess bands between the equator and the poles where liquid water would exist. But, it is quite possible the atmosphere would collapse, with gases being driven off into space at the very hot equator, and freezing solid on the ground at the poles. Such a possibility faces the planets around KOI 961.

Systems with stars like our Sun present better possibilities. The “Goldilocks” conditions exist much farther out, and axial tilt is eliminated much more slowly, as our Earth is witness. Systems such as Kepler-22b are good candidates.

The conclusion drawn from these studies is that systems similar to our Solar System present the best opportunities for life.

Earth Like Planet – Kepler 22b

Kepler 22b
Planet Kepler 22b Orbiting a G-Type Sun 600 Light Years from Earth
Image Credit: NASA / Ames / JPL-Caltech

NASA announced the discovery of an Earth-like planet located 600 light years from Earth. “Just a stroll to the local gas station for a candy bar” compared to the size of the Milky Way galaxy we inhabit.

Kepler 22b
Kepler 22 System and the Inhabitable Zone
Image Credit: NASA / Ames / JPL-Caltech


NASA has announced that the Kepler Mission has released the first 43 days of science data on more than 156,000 stars.

Kepler is designed to continuously monitor a star field in the constellations Cygnus and Lyra. The objective is to find Earth like planets circling solar-like stars in what is known as the Habitable Zone, the region around a star where water can exist as a liquid. Since planetary orbits in this region take about a year to complete, the Kepler mission is designed to last through November 2012. This will allow Kepler to catch a planet fitting the description making three transits of its star during the three and a half year mission. Kepler was launched 6 March 2009.

When a planet transits a star, it blocks some of the light from the star. If an observatory is looking at the star at just the right moment, it will see the star dim and then brighten again when the planet finishes passing in front of the star. The effect is very small. A transit will dim the starlight by 100 parts per million, and last from 2 to 16 hours if the planet is in the habitable zone.

Kepler is designed to monitor 100,000 stars continuously for three and a half years. This is enough time to capture three transits by a planet. Kepler has only one instrument on board. This is a 0.95 meter telescope equipped with a photometer. The telescope has a large field of view in order to capture all the stars simultaneously. The field of view is about the size of your hand held at arms length (105 square degrees).

Data from the Kepler mission is archived at Space Telescope Science Institute. The three missions in which the Institute participates are Hubble, the James Webb Space Telescope, and Kepler.

Kepler Diagram
Diagram of the Kepler Spacecraft
Image Credit: NASA

Kepler Photometer
Kepler Photometer Focal Plane Assembly
Image Credit: Ball Aerospace

The photometer (left) is a single instrument, composed of 42 Charge Coupled Devices (CCDs). Each of the square units in the image are two 25 mm x 50 mm CCDs, each comprised of 2200×1024 pixels.

The CCDs are read every three (3) seconds and the data is integrated over 30 minutes. The instrument has the ability to detect an earth sized object transiting a star in 6.5 hours of integrated data.

The instrument has a spectral bandpass from 400 nm to 850 nm. Data from the individual pixels that make up each star of the 100,000 main-sequence stars are recorded continuously and simultaneously. The data are stored on the spacecraft and transmitted to the ground about once per month.

The targeted region of space and the star field in the field of view can be seen here.

At right is the Kepler spacecraft during construction. The size of the Solar Array and the Telescope (wrapped in gold foil) can be seen compared to the technician working on the Solar Array.

The scientific objective of the Kepler Mission is to explore the structure and diversity of planetary systems. This is achieved by surveying a large sample of stars to:

  • Determine the percentage of terrestrial and larger planets that are in or near the habitable zone of a wide variety of stars
  • Determine the distribution of sizes and shapes of the orbits of these planets
  • Estimate how many planets there are in multiple-star systems
  • Determine the variety of orbit sizes and planet reflectivities, sizes, masses and densities of short-period giant planets
  • Identify additional members of each discovered planetary system using other techniques
  • Determine the properties of those stars that harbor planetary systems

Kepler with Solar Array (interior)
Image Credit: NASA and Ball Aerospace

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