can i let the sphere follow a point(x.y. or.z) on plane?

hi

i am beginner in unity.

how can i let the sphere follow a point on plane? in this case center of face detection. look at the picture.
thx

what i wanna do is that sphere follows center of face everytime. and as you told me yes just on the plane.

i have face.x and face.y position information but it is just on plane limited.

here is part of code

private void faceDetector()
{
	currentFrameBgr = cvCapture.QueryFrame().ToImage<Bgr, byte>();

	Texture2D tex = new Texture2D(640, 480);

	if (currentFrameBgr != null)
	{
		Image<Gray, byte> grayFrame = currentFrameBgr.Convert<Gray, byte>();

		var faces = _cascadeClassifier.DetectMultiScale(grayFrame, 1.1, 10, new System.Drawing.Size(frameWidth/8, frameHeight/8));

		foreach (var face in faces)
		{
			currentFrameBgr.Draw(face, new Bgr(0, 255, 0), 3);
			FaceCenterPointX = face.X;
			FaceCenterPointY = face.Y;

		}

		//Convert this image into Bitmap, the pixel values are copied over to the Bitmap
		currentFrameBgr.ToBitmap();

		MemoryStream memstream = new MemoryStream();
		currentFrameBgr.Bitmap.Save(memstream, currentFrameBgr.Bitmap.RawFormat);

		tex.LoadImage(memstream.ToArray());
		mt.mainTexture = tex;
	}
}