Planets | How Many Orbit Other Stars?

Our Sun has a multitude of objects in orbit around it. There are comets, asteroids, spacecraft, and planets. Do other stars also have planets? NASA’s Kepler spacecraft launched March 6, 2009 from Cape Canaveral Air Force Station in Florida. The first planet-hunting spacecraft set out on a mission to look for other planets around stars. Astronomers hoped to answer two questions.

  1. What fraction of the stars have planets in orbit around them?
  2. What fraction of those are in the habitable, or Goldilocks, zone where conditions are just right to possibly allow life to exist?

Based on ground telescopic views, large hot Jupiter-sized and large cold icy planets have already been observed around stars. Kepler is looking for those planets known as terrestrial, like Earth, that orbit at a distance that isn’t too hot or too cold. They tend to be smaller and more difficult to detect. A satellite spacecraft above our atmosphere is needed.

What Has Kepler Seen?

Kepler has stared at the same patch of sky for 3.5 yrs. The patch is small. Hold your hand out with your extended arm. That is how big the patch is. It contains 100,000 stars which were monitored continuously. If a planet in orbit around that star passed between the star and us, the level of brightness seen would decrease slightly. That is called a transit.

As of November 2013, Kepler found more than 3,500 potential planet candidates. About 200 have been confirmed. Click on a star named in the confirmation table at that link to see the details of the orbit and transit data. Interesting stuff. The candidates come in a variety of sizes around stars of different sizes. There are quite a few Earth sized candidates. But, fewer than 10 of those have had their data confirmed.


Based upon statistical analysis, most stars in the sky have planets. Furthermore, one in five stars like the Sun is home to a planet up to twice the size of Earth, orbiting in a temperate environment. Those are very encouraging numbers.

Kepler is Broken

The spacecraft launched with 4 small spinning gyroscope-like wheels attached to it. Changing the spin rate of these wheels allowed astronomers to keep the spacecraft pointed accurately at the field of 100,000 stars. Two of those reaction wheels have stopped working. The remaining two cannot keep the spacecraft pointed well enough. The mission is in danger of ending.

Recently, the astronomy team devised a work-around solution that should allow Kepler to resume studying stars. The reaction wheels were needed to counter-balance the effect of solar pressure on the spacecraft which tended to make it roll and point off target. The plan it to orient the spacecraft so the solar pressure is balanced on the ridge of the solar panel, seen in blue here and in the image at the top of this post. It is hoped the two remaining reaction wheels can maintain this alignment for 83 day stretches. Then, the spacecraft has to be repointed to prevent sunlight from entering the sensitive detector.

The plan has one serious tradeoff. It would no longer study the patch of stars it has been viewing for 3.5 yrs. A new patch would be in view each 83 days. But, it would be able to continue gathering data. The final funding decision is expected by the summer of 2014.

At this point in time, there remains over a year of original Kepler data that still needs to be reviewed and analyzed. With that, and the prospect of this new Kepler 2 mission, the spacecraft might still function well into the future. The list of Goldilocks planets will continue to grow.


4 thoughts on “Planets | How Many Orbit Other Stars?

    • No word on that. It will likely be a long time if we ever find out, too. They are so small and hard to detect compared to planets. But, it they have some advanced technology that can beam signals we can see, maybe we could find out sooner.

      Good question.

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