More Planets than Stars – But Axial Tilt may be 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.

The Astronomy and Astrophysics Decadal Survey – 2012 to 2021

by Dave Fischer

The National Research Council has released Its sixth decadal survey of astronomy and astrophysics. The plan focuses on three science objectives:

  • The exploration of the origins of the universe
  • The search for habitable planets outside our solar system
  • The use of astronomical observations to investigate fundamental physics.

The report addresses large, medium, and small activities in these fields. It surveys the existing facilities and the new facilities that would be needed, both ground based and space based. It looks at the known science objectives and where new discoveries might be made. And it looks at the promise of the proposals and the risks associated with each.

The large scale space-based projects are:

  • 1. Wide Field InfraRed Survey Telescope (WFIRST)
  • 2. Explorer Program Augmentation
  • 3. Laser Interferometer Space Antenna (LISA)
  • 4. International X-ray Observatory (IXO)

WFIRST, the near infrared wide-field telescope, is intended to explore Dark Energy and Exoplanet statistics as well as support guest survey investigations.

The Explorer Program is ongoing and funds rapid, targeted projects that deliver a high science return. Past projects include WMAP, Swift, GALEX and WISE.

LISA will exploit the new field of astronomy using long wavelength gravitational radiation measured by three spacecraft 5 million kilometers apart.

IXO is a large area, high spectral resolution x-ray observatory to explore hottest regions in the universe, including clusters of galaxies, the intergalactic medium, and black hole accretion disks.

The medium scale space-based projects are:

  • 1. New Worlds Technology Development Program
  • 2. Inflation Technology Development Program

The New Worlds technology development program is a research program to obtain preliminary observations in order to study nearby habitable planets. Included is technology development in order to make an informed decision in the second half of this decade on a flagship mission.

The Inflation Technology development program will use ground based microwave background telescopes to examine “B-mode polarization.” This is a sensitive signature of processes thought to have occurred during the earliest moments of the universe. If a signal is seen, then a space-based mission with at least ten times greater sensitivity is warranted and associated technology development would be needed.

There are four large scale ground-based projects recommended, and prioritized as follows:

  • 1. Large Synoptic Survey Telescope (LSST)
  • 2. Mid-Scale Innovations Program
  • 3. Giant Segmented Mirror Telescope (GSMT)
  • 4. Atmospheric Cerenkov Telescope Array (ACTA)

The LSST project would address research such as dark energy using gravitational lensing, dark matter, Near-Earth Kuiper-belt objects, the Solar neighborhood, and transient phenomena such as gamma-ray bursts, variable stars, and supernova.

The mid-scale program would fund annual proposals to compete for funding, of which around 7 proposals would be chosen during the decade.

The Giant Segmented Mirror Telescope project suggests that NSF chose one of the two current 30 meter telescope projects (The Giant Magellan Telescope in Chile or the Thirty Meter Telescope in Hawaii) and invest in a quarter share in order to provide access for the entire US community.

The report proposes that the team responsible for the proposed US Advanced Gamma-ray Imaging System (AGIS) collaborate as a minor partner with the European Cherenkov Telescope Array (CTA).

There is one medium scale ground-based program that was recommended by the NRC Decadal Survey:

  • 1. Cerro Chajnantor Atacama Telescope (CCAT)

This 25 meter wide-field sub-millimeter telescope would work in conjunction with the Atacama Large Millimeter Array (ALMA) in Chile.

The small scale investment recommendations are:

  • Target work-force development (TSIP, Sub-orbital, AAG, ATP)
  • Address changing role of computation and theory (TCN)
  • Support current/upcoming facilities (Gemini, Lab Astro, TCN)
  • Develop technology for the future (NSF ATI, NASA Tech. Dev.)

Other documentation:

NASA press release

Preliminary Report

NRC committee reports.

Town Hall Slides (pdf).